Image-formation apparatus, controlling method thereof and image-formation method

ABSTRACT

An image formation apparatus is provided with a photosensitive drum, an optical writing device which writes an image on the outer surface of the photosensitive drum, a developing device which develops the image using toner an intermediate transfer belt onto which the toner image on the photosensitive drum is transferred, a rotation angle detection unit which detects the rotation angle of the photosensitive drum, an eccentric detection unit which detects the amount eccentricity of the photosensitive drum from the rotary axis of the photosensitive drum, and a correction unit which, obtains eccentricity of the photosensitive drum based on the amount eccentricity, the rotation angle and the radius of the photosensitive drum, and corrects a distortion and a color offset in the toner image based on the obtained eccentricity.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. application Ser. No.10/846,705 filed on May 17, 2004 now U.S. Pat. No. 6,931,227, which inturn is a divisional of U.S. application Ser. No. 10/308,062 filed Dec.3, 2002, now U.S. Pat. No. 6,760,559, which in turn is a divisional ofU.S. application Ser. No. 09/816,456 filed Mar. 26, 2001, now U.S. Pat.No. 6,507,713, and in turn claims priority to JP 2000-86014 filed onMar. 27, 2000, JP 2000-156933 filed on May 26, 2000, JP 2000-169516filed on Jun. 6, 2000, and JP 2000-204531 filed on Jul. 6, 2000, theentire contents of each of which are hereby incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to an image formation apparatus and acontrol method thereof, and, more particularly, concerns an imageformation apparatus that is provided with a rotary member and belts,such as a belt for transporting paper toward the rotary member or atransfer belt for allowing an image formed on the surface of the rotarymember to be transferred on its own surface, and a control method ofsuch an apparatus. Moreover, the present invention also relates to animage formation apparatus provided with a photosensitive drum and abelt, such as a transporting belt for transporting paper or anintermediate transfer belt, and an image formation method of such anapparatus, and a controlling device that is applicable to a mechanismthat moves both a rotary member, which normally has eccentricities ordeviations in its diameter, and a belt in an integral manner.

BACKGROUND OF THE INVENTION

In recent years, there is an ever-increasing demand for color imageformation apparatuses capable of forming color images. One type of imageformation apparatuses capable of forming color images at high speeds hasan electrophotographic system of a tandem type. With respect toconventional image formation apparatuses of the tandem type, forexample, the inventions disclosed in Japanese Patent ApplicationLaid-Open Nos. 63-81373 and 10-246995 have been known.

Each of the inventions disclosed in the above-mentioned publications,Japanese Patent Application Laid-Open Nos. 63-81373 and 10-246995, hasfour photosensitive drums each of which has a scanning unit for applyinga laser light beam to each of the four photosensitive drums so as towrite a latent image thereon.

The four photosensitive drums are placed in parallel with each other inthe transporting direction of the paper, which is transported by thetransporting belt. Each of these is scanned (main-scanning process) inthe direction of the rotary axis by a laser light beam directed from thescanning unit, while being rotated, so that a latent image is writtenthereon. Here, one line of the latent image written by one main-scanningprocess is hereinafter referred to as a scanning line.

On the surfaces of the four photosensitive drums bearing the latentimages written thereon are supplied toners having respective colors of Y(yellow), M (magenta), C (cyan) and K (black) so as to adhere to therespective latent images. Thus, a toner image having one of the colorsis formed on each of the surfaces of the four photosensitive drums. Asheet of paper is successively pressed onto the four photosensitivedrums on which toner images have been formed. Consequently, the tonerimages of the respective colors are successively transferred the sheetof paper to form a color image.

In this case, if there is an offset between the scanning linesconstituting the toner images of the respective colors in the colorimage thus formed, a so-called color offset will occur in the colorimage, resulting in degradation in the image quality. In order toprevent the color-offset, Japanese Patent Application Laid-Open No.10-246955 has proposed an arrangement in which the photosensitive drumsare designed to rotate freely, while an annular transport belt is drivento rotate by a motor so that the transport belt is made to contact thephotosensitive drums by press-contact rollers installed below thetransport belt. The four photosensitive drums are driven to rotate,following the transport belt. At this time, the four photosensitivedrums are subjected to the same rotary force so that they are allowed torotate at the same angular velocity, thereby making it possible to forma color image that is free from positional offsets between the scanninglines.

However, in image formation apparatuses such as printers and copyingmachines, there will be ever-increasing demands for high resolution(1200 dpi or more).

In contrast, the technique disclosed in Japanese Patent ApplicationLaid-Open No. 10-246995 fails to meet these demands. In other words, inthe technique disclosed in Japanese Patent Application Laid-Open No.10-246995, the image distortion in the sub-scanning direction due to theeccentricity of the rotary axis of the photosensitive drum is correctedby detecting a outer surface dislocation and using the dislocationinformation continuously. However, when a high resolution is required,the actual amount of correction tends to deviate from the dislocationinformation in their correlation as the resolution becomes higher.

Moreover, in general, the photosensitive drum tends to have a slighteccentricity due to the limitation in its assembling precision. FIG. 43is a drawing that shows a state in which a transfer belt is made tocontact a photosensitive drum 1801 having such an eccentricity by acontact roller. The photosensitive drum 1801 shown in this Figure has across-section that is orthogonal to the rotary axis perpendicular to thepaper surface, that passes through point O.

The photosensitive drum 1801 having the eccentricity rotates centered onthe center axis passing through point O. In contrast, a transport belt1802, which has an annular shape, is allowed to move in the direction ofarrow A. A contact roller 1803 is made to contact the transport belt1802 from below while being supported by a spring 1804 so that thetransport belt 1802 is made to press-contact the photosensitive drum1801. A sheet of paper, not shown, is made to press-contact thephotosensitive drum 1801 by the press-contact roller 1803 through thetransport belt 1802. Thus, a toner image formed on the surface of thephotosensitive drum 1801 is transferred onto the sheet of paper.

The distance from point O to the outer surface of the photosensitivedrum 1801 having the eccentricity varies depending on the angle ofrotation when observed at a fixed point. For this reason, when thecenter of gravity of the photosensitive drum 1801 is located at G₁, thepaper and the photosensitive drum 1801 are in contact with each other atpress-contact position P₁, while when the center of gravity of thephotosensitive drum 1801 is located at G₂, they are in contact with eachother at press-contact position P₂.

The press-contact roller 1803 is allowed to move up and down to acertain degree since this is supported by a spring 1804. Since thepress-contact roller 1803 also has an eccentricity, the press-contactposition varies in a complex manner, thereby giving greater adverseeffects to the angular velocity of the photosensitive drum 1801.

When the angular velocity of the photosensitive drum 1801 varies, thedistance between scanning lines of a latent image to be written on thephotosensitive drum 1801 becomes irregular, resulting in a distortion inthe image to be formed. Moreover, in the case of a color image formationapparatus of the tandem type having a plurality of photosensitive drumsin which multi-color toner images are superposed so as to form a colorimage, if the angular velocities of the photosensitive drums deviate,offsets occur in the transferring positions of the toner images of therespective colors, resulting in degradation in the image quality of animage to be formed.

Here, another arrangement has been proposed in which: the transferringposition of a toner image is estimated through calculations, and basedupon the results thereof, the image forming conditions are adjusted soas to make the transferring positions coincident with each other.However, when the angular velocity of the photosensitive drum 1801varies, it becomes difficult to accurately estimate the transferringpositions of the toner images, resulting in failure to adjust thetransferring positions of the toner images through adjustments of theimage formation apparatus. Therefore, in the image formation apparatusin which the angular velocity of the photosensitive drum varies, it isimpossible to improve the image quality by eliminating the offsets inthe transferring positions of the toner images through the adjustmentsof the image forming conditions.

Moreover, the color image formation apparatus of the tandem type isprovided with a writing unit for each of the photosensitive drums. Here,the writing timings and properties of the optical systems in therespective writing units placed in the respective photosensitive drumsare not necessarily coincident with each other. For this reason, thewriting timing deviates for each of the photosensitive drums, and evenwhen there is no eccentricity in each of the photosensitive drums, theremight be deviations in the transferring positions for the toner imagesof the respective colors.

Furthermore, the radius of each of the photosensitive drums tends tohave a slight deviation due to the limitation in the processingprecision. In this case also, deviations tend to occur in thetransferring positions in the toner images of the respective colors,regardless of the eccentricity of each of the photosensitive drums.

In the future, along with the ever-increasing demands for high-qualityprinting with high resolution (1200 dpi or more), very high-precisionphotosensitive drums have to be produced in order to solve theabove-mentioned problems. Taking into consideration the development ofthe technology in the future, it will be possible to improve theprocessing precision of the photosensitive drum to a certain extent;however, there will be a limitation in the improvement of the processingprecision.

SUMMARY OF THE INVENTION

One of the objectives of the present invention is to provide an imageformation apparatus which, even when high resolution is required forforming an image, sufficiently prevents a distortion and a color offsetin the sub-scanning direction of an image, and makes deviations in thesub-scanning pitch due to the eccentricity of the photosensitive drumless conspicuous so that image data in the sub-scanning direction isoutputted in the same timing as the case having neither eccentricity inthe photosensitive drum nor deviations in the drum, and makes itpossible to cut production costs and also to form correction data forcorrecting distortions and color offsets in the image in thesub-scanning direction with high precision.

Moreover, another objective of the present invention is to provide animage formation apparatus and a control method for the image formationapparatus, in which, independent of the states of respective imageformation apparatuses such as eccentricity due to deviations at the timeof assembling photosensitive drums, deviations in the writing timing ofwriting units and deviations in the radii of the photosensitive drums,toner images of the respective colors are transferred on a sheet ofpaper on the transfer belt without positional offsets, thereby making itpossible to form high-quality images.

Furthermore, still another objective of the present invention is toprovide an image formation apparatus and a control method for the imageformation apparatus, in which independent of the states of respectiveimage formation apparatuses such as eccentricity due to deviations atthe time of assembling photosensitive drums and deviations in the radiiof the photosensitive drums, toner images of the respective colors aretransferred on a sheet of paper on the transfer belt without positionaloffsets, thereby making it possible to form high-quality images.

An image formation apparatus in accordance with the present invention isprovided with: a photosensitive drum, an optical writing device whichcarries out an optical writing process in the main-scanning direction onthe outer surface of the photosensitive drum at least line by line, adeveloping device which develops an electrostatic latent image opticallywritten on the photosensitive drum by using toner, a transport memberthat is allowed to move in synchronism with the photosensitive drumwhile being pressed onto the photosensitive drum to transport a sheet ofpaper so that the toner image on the photosensitive drum is transferredon the sheet, or an intermediate transfer belt which transfers the tonerimage located on the photosensitive drum, that is shifted in synchronismwith the photosensitive drum while being pressed onto the photosensitivedrum, a rotation angle detection unit which detects the rotation angleof the photosensitive drum directly or indirectly, an eccentricitydetection unit which detects the eccentric position from the rotary axisof the photosensitive drum located in the center of the circlecross-section of the photosensitive drum, and a correction unit which,based upon the results of detection by the rotation angle detection unitand the eccentricity detection unit, finds the amount of eccentricity ofthe photosensitive drum, the eccentric rotation angle and the radius ofthe photosensitive drum, and based upon the resulting values, corrects adistortion and a color offset in the toner image that has beentransferred on the sheet or the intermediate transfer belt. With thisarrangement, based upon the amount of eccentricity of the photosensitivedrum, the eccentric rotation angle and the radius of the photosensitivedrum, the distortion and color offset in the toner image that has beentransferred are corrected so that, even when high resolution is requiredfor forming an image, it is possible to sufficiently prevent thedistortion and color offset in the sub-scanning direction of the image.

Moreover, another image formation apparatus in accordance with thepresent invention is provided with: at least one rotary member such as aphotosensitive drum that is pressed onto the belt directly or indirectlyand allowed to integrally rotate with the shift of the belt; a beltdriving unit which moves the belt, or a rotary member driving unit whichrotates the rotary member such as the photosensitive drum; and at leastone of a rotary member load correction unit for correcting variations inthe load imposed on the rotary member such as the photosensitive drumand a belt load correction unit for correcting variations in the loadimposed on the belt. With this arrangement, it is possible to regulatethe variations in the load imposed on the rotary member such as thephotosensitive drum or the belt, and consequently to reduce the amountof the load variations transmitted to the belt such as the transportbelt or the intermediate transfer belt; therefore, it becomes possibleto prevent a slip from occurring between the belt or the sheet of paperand the rotary member such as the photosensitive drum.

Moreover, a control method for an image formation apparatus inaccordance with the present invention, which is a control method for animage formation apparatus that is provided with a plurality ofphotosensitive drums for forming images, has at least one of aneccentricity detection step of detecting the eccentricity of eachphotosensitive drum, a measuring step of measuring the radius of eachphotosensitive drum and a distance detection step of detecting thedistance between the photosensitive drums, and a detection function,placed at a certain portion thereof, for detecting the rotation angle ofeach photosensitive drum or for detecting the shift of the belt; thus,the positioning of the rotation angle is carried out independently foreach photosensitive drum. With this arrangement, although variations indensity occur in each latent image formed on the photosensitive drum dueto the eccentricity of each photosensitive drum, a toner image, formedon a sheet of paper or the intermediate transfer belt by superposing thelatent images, has a state of variations in density that are virtuallymade coincident with each other; thus, it becomes possible to obtain ahigh-quality image.

Moreover, still another image formation apparatus in accordance with thepresent invention, which is an image formation apparatus that uses aplurality of photosensitive drums for forming images, has at least oneof an eccentricity detection unit which detects the eccentricity of eachphotosensitive drum, a measuring unit which measures the radius of eachphotosensitive drum and a distance detection unit which detects thedistance between the photosensitive drums, and a detection unit, placedat a certain portion thereof, for detecting the rotation angle of eachphotosensitive drum or for detecting the shift of the belt; thus, thepositioning of the rotation angle is carried out independently for eachphotosensitive drum. With this arrangement, although variations indensity occur in each latent image formed on the photosensitive drum dueto the eccentricity of each photosensitive drum, a toner image, formedon a sheet of paper or the intermediate transfer belt by superposing thelatent images, has a state of variations in density that are virtuallymade coincident with each other; thus, it becomes possible to obtain ahigh-quality image.

Another control method for an image formation apparatus of the presentinvention, which is a control method for an image formation apparatushaving at least one rotary member such as a photosensitive drum that ispressed onto the belt directly or indirectly and allowed to integrallyrotate with the shift of the belt; and a belt driving unit which movesthe belt, or a rotary member driving unit which rotates the rotarymember such as the photosensitive drum, is provided with at least one ofa rotary member load correction step of correcting variations in theload imposed on the rotary member such as the photosensitive drum and abelt load correction step of correcting variations in the load imposedon the belt. With this arrangement, it is possible to regulate thevariations in the load imposed on the rotary member such as thephotosensitive drum or the belt, and consequently to reduce the amountof the load variations transmitted to the belt such as the transportbelt or the intermediate transfer belt; therefore, it becomes possibleto prevent a slip from occurring between the belt or the sheet of paperand the rotary member such as the photosensitive drum.

Moreover, still another image formation apparatus in accordance with thepresent invention, which is an image formation apparatus having at leastone rotary member that is pressed onto a belt directly or indirectly andallowed to rotate following the shift of the belt and a driving rollerfor driving the belt, is provided with a driving-roller drive unit whichdrives the driving roller, a rotary-member driving unit which drives therotary member, a load variation detection unit which detects loadvariations in the belt and a control unit which controls the operationof the driving-roller drive unit or the rotary-member driving unit inaccordance with variations in the load of the belt. The presentinvention is supposed to be used for an image formation apparatusprovided with at least a rotary member that is pressed onto an annularbelt for transporting, for example, a sheet of copy paper, directly orindirectly, with the paper interpolated in between, and allowed to movefollowing the shift of the belt, and a driving roller for driving thebelt. The driving-roller drive unit drives the belt. Moreover, therotary-member driving unit drives the rotary member. The load variationdetection unit detects the load variation imposed on the belt. Thecontrol unit controls the operations of the driving-roller drive unitand the rotary-member driving unit in accordance with variations in theload of the belt. With thins arrangement, it is possible to detect aload transmitted from the rotary member to the belt, and based upon theresults of the detection, the driving operation of the rotary member iscontrolled so as to cope with the entire load imposed on the drivingroller, thereby making it possible to eliminate a slip on the belt.Therefore, the image formation apparatus is readily applied to ahigh-quality printing operation.

Furthermore, still another image formation apparatus in accordance withthe present invention, which is provided with at least a rotary memberthat rotates while being pressed onto the belt directly or indirectly, avelocity detection unit which detects at least one of the shiftingvelocity of the belt and the velocity related to the rotary member, anda control unit, and in the system having the velocity detection unit andthe control unit, a velocity setting unit which sets the shiftingvelocity of the belt and the rotation velocity of the rotary member tovelocities that allow the belt and the rotary member to move integrallyis further installed. With this arrangement, even when there is avariation in the shape of each rotary member, it is possible to preventa slip from occurring between the rotary member and the belt, betweenrotary member and the sheet of paper, or between the sheet of paper andthe belt.

Moreover, still another image formation method in accordance with thepresent invention, which is an image formation method applied to animage formation apparatus having at least one rotary member that ispressed onto a belt directly or indirectly and allowed to rotatefollowing the shift of the belt and a driving roller for driving thebelt, is provided with a driving-roller driving step for driving thedriving roller, a rotary-member driving step for driving the rotarymember, a load variation detection step which detects load variations inthe belt and a control step for controlling the operation of the drivingroller or the rotary member in accordance with variations in the load ofthe belt. The present invention relates to an image formation method forcontrolling an image formation apparatus provided with at least a rotarymember that is pressed onto an annular belt for transporting, forexample, a sheet of copy paper, directly or indirectly, with the paperinterpolated in between, and allowed to move following the shift of thebelt, and a driving roller for driving the belt. The driving-rollerdriving step drives the belt. Moreover, the rotary-member driving stepdrives the rotary member. The load variation detection step detects theload variation imposed on the belt. The control step controls theoperations of the driving roller and the rotary member in accordancewith variations in the load of the belt. With this arrangement, it ispossible to detect a load transmitted from the rotary member to thebelt, and based upon the results of the detection, the driving operationof the rotary member is controlled so as to cope with the entire loadimposed on the driving roller, thereby making it possible to eliminate aslip on the belt. Therefore, the image formation apparatus is readilyapplied to a high-quality printing operation.

Furthermore, still another image formation method in accordance with thepresent invention, which is an image formation method for controlling atleast a rotary member that rotates while being pressed onto the beltdirectly or indirectly, is provided with a velocity detection step whichdetects at least one of the shifting velocity of the belt and thevelocity related to the rotary member, and a control step, and in thesystem having the velocity detection step and the control step, avelocity setting step for setting the shifting velocity of the belt andthe rotation velocity of the rotary member to velocities that allow thebelt and the rotary member to move integrally is further provided. Withthis arrangement, even when there is a variation in the shape of eachrotary member, it is possible to prevent a slip from occurring betweenthe rotary member and the belt, between rotary member and the sheet ofpaper, or between the sheet of paper and the belt.

Other objects and features of this invention will become apparent fromthe following description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual drawing that shows the schematic structure of animage formation apparatus in accordance with the present invention;

FIG. 2 is a front view that shows the structure of a tension roller inthe image formation apparatus;

FIG. 3 is a block diagram that shows an electrical connection of acontrol system in the image formation apparatus;

FIG. 4 is an explanatory drawing that explains the relationship betweenthe shifting velocity of a transport belt and the rotation angularvelocity of a photosensitive drum in the image formation apparatus;

FIG. 5 is an explanatory drawing that explains the relationship betweenthe shifting velocity of a transport belt and the rotation angularvelocity of a photosensitive drum in a conventional image formationapparatus;

FIG. 6 is a drawing that explains an eccentric position detecting unitwhich detects the eccentric position of the photosensitive drum of theimage formation apparatus;

FIG. 7 is a drawing that explains the an eccentric position detectingunit which detects the eccentric position of the photosensitive drum ofthe image formation apparatus;

FIG. 8 is a drawing that explains a means for finding the angle from anexposing position to a transferring position in the image formationapparatus;

FIG. 9 is a plan view that shows respective photosensitive drums and atransport belt in the image formation apparatus;

FIG. 10 is an explanatory drawing that shows a means for outputtingimage data in the image formation apparatus;

FIG. 11 is an explanatory drawing that shows the means for outputtingimage data in the image formation apparatus;

FIG. 12 is an explanatory drawing that shows the means for outputtingimage data in the image formation apparatus;

FIG. 13 is an explanatory drawing that shows the means for outputtingimage data in the image formation apparatus;

FIG. 14 is an explanatory drawing that shows the means for outputtingimage data in the image formation apparatus;

FIG. 15 is an explanatory drawing that shows the relationship between atest mark that is exposed by each photosensitive drum and formed on abelt and a reference mark formed on the belt;

FIG. 16 is a block diagram that shows a schematic structure of the imageformation apparatus in accordance with the present invention;

FIG. 17 is a drawing that explains an essential portion of the imageformation apparatus in accordance with the present invention;

FIG. 18 is a drawing that explains the tension roller shown in FIG. 17;

FIG. 19 is a drawing that shows the photosensitive drum of the presentinvention as a model;

FIG. 20 is a drawing that explains a process in which the transferringposition of a latent image written on a photosensitive drum having aneccentricity;

FIG. 21 is a drawing that explains a structure for detecting the radius,eccentric position and rotation angle of the photosensitive drum in theimage formation apparatus of the present invention;

FIG. 22 is a drawing that shows a reference mark, and a toner image thatis formed by each photosensitive drum based upon the reference mark;

FIG. 23 is a block diagram that explains a structure for controlling thestructure shown in FIG. 21;

FIG. 24 is a flow chart that explains the detection of the state of theimage formation apparatus and the adjustment on image forming conditionsthat are carried out in the image formation apparatus of the presentinvention;

FIG. 25 is a drawing that explains an essential portion of the imageformation apparatus of the present invention;

FIG. 26 is a drawing that explains a load variation correcting motorshown in FIG. 25;

FIG. 27 is a block diagram that explains the control of the imageformation apparatus executed by a rotation angle detecting encoder and amotor 907;

FIG. 28 is a block diagram that explains a structure for controlling theload variation correcting motor;

FIG. 29 is a drawing that shows another structural example of the loadvariation correcting motor;

FIG. 30 is a drawing that shows still another structural example of theload variation correcting motor;

FIG. 31 is a drawing that shows the other structural example of the loadvariation correcting motor;

FIG. 32 is a drawing that shows a structural example of a flywheel;

FIG. 33 is a drawing that shows another structural example of an imageformation apparatus in accordance with the present invention;

FIG. 34 is a drawing that explains a step of adjusting a line densityvariation position of a latent image formed on each photosensitive drum;

FIG. 35 is a drawing that explains a method for detecting the rotationangle of the photosensitive drum;

FIG. 36 is a drawing that explains an essential portion of the imageformation apparatus of the present invention;

FIG. 37 is a block diagram that explains the structure of a drum loadvariation correcting control section that controls the load variation ofthe photosensitive drum;

FIG. 38 is a block diagram that explains the structure of an improveddrum load variation correcting control section that controls the loadvariation of the photosensitive drum;.

FIG. 39 is a block diagram that explains an fd generation section forgenerating a clock frequency fd shown in FIG. 38;

FIG. 40 is a block diagram that explains a structure for controlling amotor of the image formation apparatus of the present invention;

FIG. 41 is a block diagram that explains a circuit for detecting awaveform that is generated in a driving-roller driving motor current inaccordance with a controlling error in a load variation correctingsystem shown in FIG. 40;

FIG. 42 is a timing chart of the block diagram shown in FIG. 40 and FIG.41; and

FIG. 43 is a drawing that explains a transferring position in aconventional image formation apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description will discuss one embodiment of the presentinvention as a first embodiment. FIG. 1 is a conceptual drawing thatshows a schematic structure of an image formation apparatus inaccordance with the first embodiment of the present invention. Thisimage formation apparatus 1 is an image formation apparatus of a tandemtype for forming a color image on a sheet such as paper in theelectrophotographic system, and has four image forming sections 2, 3, 4and 5 that are aligned in a row. The image forming sections 2, 3, 4 and5 are respectively provided with photosensitive drums 6, 6, . . . .Although not shown in FIG. 1, on the periphery of each of thephotosensitive drums 6, devices having known structures for carrying outan image forming operation in the electrophotographic process, such as acharging device for charging each photosensitive drum 6, an opticalscanning unit that is an optical writing device for optically writing anelectrostatic latent image on each photosensitive drum 6, a developingdevice which develops the electrostatic latent image with toner and acleaning device for removing residual toner from each photosensitivedrum 6, are installed. The four image forming sections 2, 3, 4 and 5form images having respective colors of C (cyan), M (magenta), Y(yellow) and Bk (black) on a sheet of paper.

An endless conveyor belt 7 serving as a transport member is placed belowthe image forming sections 2, 3, 4 and 5. This conveyor belt 7 isallowed to pass over a driving roller 8, a driven roller 9 and a tensionroller 10. The conveyor 7, which is driven by a driving roller 8, formsa transport path on which a sheet of paper is successively transportedto the image forming sections 2, 3, 4 and 5.

As illustrated in FIG. 2, the tension roller 10, which is supported onone end of a shaft 11 so as to freely rotate thereon, with the shaft 11being freely rotatably attached to the frame of the image formationapparatus 1, is pressed by a spring 12 so that it is possible to preventthe conveyor belt 7 from slackening and also to press the conveyor belt7 onto photosensitive drums 6, 6, . . . along their tangential lines. Inorder to assist the press-contact between the endless conveyor belt 7forming the paper transport path and the photosensitive drums 6, 6, . .. , press-contact rollers 13, 13, . . . are placed between therespective photosensitive drums 6,6. The press-contact rollers 13, 13, .. . are supported so as to freely rotate, and pressed by a spring so asto be pressed against the conveyor belt 7. Thus, when the driving roller8 is rotated by a driving motor, not shown, the conveyor belt 7 istransported at a constant velocity, the sheet of paper on the conveyorbelt 7 is transported at a constant velocity, and the photosensitivedrums 6, 6, . . . are driven by the conveyor belt 7 to rotate.

At press-contact portions between the conveyor belt 7 and thephotosensitive drums 6, 6, . . . below the conveyor belt 7, transferringcorona chargers 14, 14, . . . each of which transfers a toner image ofeach photosensitive drum 6 onto the sheet of paper when the sheet ofpaper on the conveyor belt 7 is located below the respectivephotosensitive drums 6, 6, . . . are placed.

FIG. 3 is a block diagram that shows electrical connections in a controlsystem of the image formation apparatus 1. As illustrated in FIG. 3, inthis control system, a CPU 15 for carrying out various calculations andfor controlling the respective parts in a concentrated manner, a ROM 16for storing fixed data such as various control programs and a RAM 17 forproviding a work area for the CPU 15 are connected by a bus 18. Varioussensors 19 and various actuators 20 are connected to the bus 18. Thevarious sensors 19 include a linear encoder 22, which will be describedlater, and various detectors such as an exposure starting positiondetector 24, a paper leading position passage detector 25 and areference position error detector 26, which will be described later. Thevarious actuators 20 include motors for driving the photosensitive drum6, the driving roller 8 and the press-contact rollers 13, which will bedescribed later.

Referring to the following items 1 to 9, an explanation will be given ofthe idea of a controlling operation of the image formation apparatus 1that is realized by the CPU 15 following a control program stored in theROM 16.

1. Concerning the Shifting Velocity of the Conveyor Belt 7 and theRotation Angular Velocity of the Photosensitive Drum 6

First, an explanation will be given of the relationship between theshifting velocity of the conveyor belt 7 and the rotation angularvelocity of the photosensitive drum 6. Based on this, a rotation angledetection unit is realized.

The structure in FIG. 1 can be shown as a model as illustrated in FIG.4. In FIG. 4, supposing that ε is an amount of eccentricity and θ is anangle of the eccentric position with respect to the x-axis, the shiftingvelocity of a contact point T between the conveyor belt 7 (hereinafter,referred to simply as “belt”) and the photosensitive drum 6(hereinafter, referred to simply as “drum”) is represented ascoordinates as follows:(−ε sin θ·ω, ε cos θ·ω), ω=dθ/dt  (1).

Therefore, the velocity Vs in the rotation direction S around the drumrotation center O is represented by the following equation:Vs=V cos α−ε sin θ·ω·cos α+ε cos θ·ω·sin α  (2)Here, V is the belt shifting velocity, and α represents an angle made bythe belt and a line that is orthogonal to a line r connecting the drumrotation center and the contact point at the position of the contactpoint T.

Therefore, the following equation is satisfied:ω=Vs/r=(V cos α−ε sin θ·ω·cos α+ε cos θ·ω·sin α)/r  (3)Then, from the general formula of cosine, the following equation holds:$\begin{matrix}\begin{matrix}{r^{2} = {R^{2} + ɛ^{2} - {2R\; ɛ\;{\cos\left( {{\pi/2} - \theta} \right)}}}} \\{= {R^{2} + ɛ^{2} - {2R\;{ɛsin\theta}}}}\end{matrix} & (4)\end{matrix}$Here, R represents the drum radius.

From the theorem of sine, the following equations hold: $\begin{matrix}\begin{matrix}{{{ɛ/\sin}\;\alpha} = {r/{\sin\left( {{\pi/2} - \theta} \right)}}} \\{= {{r/\cos}\;\theta}}\end{matrix} & (5) \\{{{\sin\;\alpha} = {{ɛcos\theta}/r}},{{\cos\;\alpha} = {\left( {R - {ɛsin\theta}} \right)/r}}} & (6)\end{matrix}$Here, substituting (4) and (5) to (3) forms:ω={VR−(V+ωR)ε sin θ+ωε²}/(R ²+ε²−2Rε sin θ),Therefore, the following equation holds:ω(R ²+ε²−2Rε sin θ)=VR−(V+ωR)ε sin θ+ωε²V=Rω  (7)

As clearly shown by the above description, even in the case when thereis an eccentricity, if the belt shifting velocity V is constant withouta slip, the rotation angular velocity of the drum is made constant inthe same manner.

Therefore, when a detector for detecting the movement or the absoluteposition of the belt (for example, as illustrated in FIG. 9, timingmarks 21 having constant intervals are put on the edge of the belt onwhich no paper passes (portions outside the area indicated by symbol 7a) with a reference mark 23 being also put on the belt, and these marks21 and 23 are detected so that the absolute position is recognized by alinear encoder 22, a leading position detector 25, etc.) is installed,it is possible to detect the reference position of the drum rotation(the detection being made by outputting one pulse for each rotation),and consequently to identify the rotation angle position of the drum,without the need of a rotary encoder that is used for detecting therotation angle, and is capable of detecting the absolute position ofeach drum. Then, for example, the drum is rotated and the linear encoder22 measures one cycle of the drum rotation detected by the referenceposition detector (the detector for detecting the reference angleposition by outputting one pulse for each rotation of the photosensitivedrum) so that the rotation angle of the drum that is rotated per onepulse of the output of the linear encoder 22 is found.

In contrast, one of the four drums of the tandem type may be providedwith a detector such as a rotary encoder for measuring the absoluterotation angle with the other drums being provided with the referenceposition detectors for measuring the reference position of the rotation,and in order to detect the position of the belt, only the referenceposition of the belt may be detected; thus, it becomes possible to findthe position on the belt or the rotation angle of each of the drums. Inthis case, however, since, based upon the radius of the one selecteddrum, measurements are made on the rotation angle positions of the fourdrums and the position of the belt, the corresponding errors arise. Inother words, in this system, a slight magnification error may occur inthe transferred image with respect to the sheet of paper.

Moreover, the rotation angular velocity of each drum may be controlledso as to have a constant rotation angular velocity in accordance withthe disc radius thus, it becomes possible to eliminate a slip.

Even when there is an eccentricity in the drum, the contact portionbetween the belt and drum does not form a maximum value (apex) on thedrum circle cross-section in the direction on the belt side; however, inthe structure of the conventional image formation apparatus (see FIG.5), since the rotation velocity of the drum is not constant even whenthe belt velocity is constant, it is clear that the above-mentionedeffect is not obtained. In other words, in the structure of theconventional technique, the drum and the belt are pressed to contacteach other by a spring force of the press-contact roller so as totransmit the driving force to the drum. Since the contact portion of thebelt and the drum is located on the lower portion of the drum rotaryaxis, that is, in the proximity of the Y-axis in FIG. 4, it is clearthat the rotation angular velocity of the drum varies due to theeccentricity.

2. Detection of the Eccentric Position of the Photosensitive Drum 6

Next, an explanation will be given of the detection of the drumeccentric position. Based upon this, an eccentricity detection unit isrealized.

(1) System for Detecting Variations in the Reflection Angle

In FIG. 6, a fluctuation in the x-axis direction in the reflected lightof an exposing light beam that is made incident on the drum diagonallyto the perpendicular cross-section on the paper face including they-axis is detected by a position sensor 28 that is placed at a positionindicated by “Y=R₀+b” as shown in FIG. 6. Thus, the eccentric positioncan be measured when the drum is rotated. The rotation angle of the drumis detected by a rotary encoder, not shown, that is moved in associationwith the drum. The encoder may be a known device of a type capable ofmeasuring the absolute angle, or may be one of a type that detects byusing the aforementioned linear encoder 22, etc.

The rotation angle θ=π/2 or θ=3π/2 of the eccentric position from thex-axis can be measured in the following manner. In the case when thelight scanning width on the position sensor 28 is W, a position thatsatisfies 1/2W is detected, and this position corresponds to θ=π/2. Whenthis position is determined, each shift of rotation of π/2 detected bythe rotation angle detection encoder in accordance with the drumrotation provides a position θ=0 or θ=π. At this time, the position Xdof the beam directed onto the position sensor 28 is measured so as tofind ε.

FIG. 7 shows a state in which the eccentric position is at a position onthe x-axis, that is, θ=0.

In FIG. 7, the following equation holds:Xd=tan 2β·(R ₀ +b−E)  (8)

Here, R₀ represents the radius of an ideal drum that has an ideal shape(having no error in the drum shape), β is an incident angle of lightthat is made incident on the exposing position E in FIG. 7 andreflected.

Here, the following equations hold:ε/R=sin β  (9)tan 2β=2 sin β cos β/(cos² β−sin²β)=2(ε/R){1−(ε/R)²}^(1/2)/{1−2(ε/R)²}  (10)E(R ²−ε²)^(1/2)  (11)Xd=2(ε/R){1−(ε/R)²}^(1/2)/{1−2(ε/R)² }×[R ₀ +b−R·[1−(ε/R)²]^(1/2)]  (12)Here supposing that ε/R=ρ,  (13)4R ²ρ⁶+8RXdρ ⁵+{4Xd ²−8R ²+4(R ₀ +b)²}ρ⁴−12RXdρ ³+4{−Xd ² +R ²+(R ₀+b)²}ρ²+4RXdρ+Xd ²=0  (14)

Since R is detected by another means that will be explained below, whenXd is detected, ε is found. In fact, the solutions of equations using Rand Xd as parameters are prepared as a table in ROM 16, etc., and thistable may be looked up.

As described above, the eccentric position (θ, ε) is found from thex-axis of the drum.

(2) Dislocation Detecting System of the Surface on the PhotosensitiveDrum 6

This system, which detects the outer surface dislocation due to aneccentricity in the drum, uses a detector that is constituted by, forexample, a light-emitting element for releasing an optical beam onto thedislocation detection position on the drum outer surface, alight-receiving element for receiving the light beam reflected by thedrum (for example, two-division photodiode elements) and an opticalsystem which allows light detected on the light-receiving element tovary due to variations of the drum outer surface caused by theeccentricity (for example, an optical system using a focus-errordetection system, etc., used in an optical disk). With this arrangement,a photocurrent, which corresponds to the variation in the distancebetween the detector and the detection position, is allowed to flowthrough the light-receiving element. By detecting this, it is possibleto detect the position of the eccentricity. Moreover, when thephotosensitive drum is rotated, the zero-cross point of the change inthe output signal and the peak position are detected so that, based uponthe relationship of these and the set position of the detector, theeccentric position (θ, ε) from the x-axis is found.

In this image formation apparatus 1, it is only necessary to detectwhere the eccentric position (θ, ε) is located on the drum rotationangle. In other words, in the image formation apparatus 1, since thedrum rotation angle is detected by another means as described earlier,it is only necessary to find where the eccentric position, detected byeither of the above-mentioned two means (1) and (2), is located on therotation angle of the photosensitive member and how much amplitude ε ithas.

3. Concerning the Angle from the Exposing Position to the TransferringPosition

Next, an explanation will be given of a means for finding the angle fromthe exposing position on the drum to the transferring position of thetoner image on a sheet of paper. Based upon this, a correction unit isrealized.

In FIG. 8, (in FIG. 8, for convenience of explanation, position E, whichfaces the belt with respect to the center axis of the photosensitivedrum rotation, is selected as the exposing position), a triangle OGEindicated by a dotted line, which is determined at the time of exposure,is used for determining the transferring position. In other words, animage (indicated by a dotted line on the drum in FIG. 8), which has beenexposed at a position (here, referred to as an eccentric position) atwhich the drum center of gravity (the center of the drum circlecross-section) G makes a rotation angle θ (angle GOx), is transferred ata position (x=−s) that is dislocated from an ideal transferring position(x=0) after having been rotated with a rotation angle ΘT. Here, therotation angle ΘT from the exposure to the transfer is represented by:ΘT=π−β  (15)where β represents an angle GEO.sin β=(ε/R)cos θ  (16)ΘT=π−sin⁻¹ {(ε/R)cos θ}  (17)s indicating the transferring position is represented by:$\begin{matrix}\begin{matrix}{s = {{ɛcos}\left( {\theta - \beta} \right)}} \\{= {{{ɛcos\theta}\left( {ɛ/R} \right)}\left\lbrack {\left\lbrack {\left( {R/ɛ} \right)^{2} - {\cos^{2}\theta}} \right\rbrack^{1/2} + {\sin\;\theta}} \right\rbrack}}\end{matrix} & (18)\end{matrix}$

Based upon the above-mentioned facts, an output means for image data formodulating an exposing beam in order to correct distortion and coloroffset of an image is designed, and the description thereof will begiven later.

For example, in the case when, instead of a transferring corona charger14, a known system for transferring a toner image on paper by applyingan electric potential to a roller facing the drum is used, thetransferring position is different from s described here due todeviations in the rotation angle θ and the drum radius; however, ΘT ands are found by correcting these based upon a predetermine relationship.

4. Concerning Means for Outputting Image Data

Based upon these, a correction unit is realized.

4-1. Fixed Exposure Position System

1) Output Timing of Image Data at the Exposing Position

Output timing of the main-scanning image is adjusted in order to alwaystransfer a toner image at an optimal position. In other words, at thetime of an ideal drum diameter R₀, a toner image is transferred afterhaving been shifted by πR₀. However, when there are eccentricities inthe drum causing deviations in the drum diameter, the toner image istransferred on paper after having been shifted by a drum rotation angleof ΘT, with the result that the transferring position has an offset of−s from the ideal transferring position T. After having beentransferred, the transferred image on the belt is shifted at a speed V.Thus, the exposure data is transferred with the offset of −s from theideal transferring position T after a lapse of time ΘT/ω=τ. In otherwords, it is transferred after the belt has been shifted by a distanceof Vτ. Supposing that the ideal drum radius is R₀ and the drum rotationangular velocity at this time is ω₀, the following equation holds:V=R₀ω₀

In the ideal drum, the toner image is supposed to be transferred after alapse of time π/ω₀=τ₀. Therefore, on the belt, the image, which issupposed to be located with a shift distance x=Vτ₀ after exposure, isformed at a position x=Vτ. In other words, it is possible to form anideal image when image data corresponding to x=Vτ is outputted on theexposing side. Data, d=V (τ₀−τ) before, needs to be outputted.V=Rω=R₀ω₀  (19)ΘT=π−sin⁻¹ {(ε/R)cos θ}  (20)d=V(π/ω₀ −ΘT/ω)=R[πω/ω₀−π+sin⁻¹ {(ε/R)cos θ}]  (21)d=π(R ₀ −R)+R sin⁻¹ {(ε/R)cos θ}  (22)

In the case of no eccentricities, the image data only needs to beoutputted with an offset of d=π(R₀−R). In this case, since the drumperipheral velocity is a constant value of V, the sub-scanning pitch isconstant. Here, in the case when there are eccentricities, in accordancewith the above-mentioned equation (22), after a delay corresponding tod, the data is outputted (depending on the angle θ, the image data ispreliminarily outputted)

2) Synchronous Signal of Image Data Output

Based upon a clock synchronizing to the shift of the belt, asub-scanning synchronous signal SYs with a pitch of P/N (N: integer) ofthe scanning pitch P, is generated. For this arrangement, as illustratedin FIG. 9, timing marks 21 are formed on the belt so as to be detectedby a linear encoder 22. In the case when main-scanning image data isoutputted and exposed in synchronism with the sub-scanning pitch P, ifthe drum has an ideal shape, the sub-scanning pitch of an exposed imageonto the drum and the sub-scanning pitch of the image transferred ontothe belt are the same, and the image is transferred at an ideal positionon the belt. Here, in FIG. 9, reference number 7 a indicates an areathrough which a sheet of paper on the conveyor belt 7 passes.

Here, in the case of no eccentricities in the drum shape with a sizegreater than the ideal shape, although the sub-scanning pitch is equal,the transferring position on the belt is located ahead of the idealposition in the belt advancing direction with an advance correspondingto d_(R)=π|(R₀−R)|. Therefore, by outputting image data with the advancecorresponding to d_(R) from the timing at which the image is outputtedat the time of exposure in the case of the ideal drum, the image isformed at the same position as that of the ideal drum.

In the case when there are eccentricities, image data, which has a delaycorresponding to d (d=π(R₀−R)+R sin⁻¹ {(ε/R)cos θ}) from the timing atwhich the image is outputted at the time of exposure in the case of theideal drum, is outputted. The rotation angle θ at the eccentric positionis detected by, for example, a rotary encoder, not shown, connected tothe drum axis.

3) Detection of Eccentricity ε and Drum Radius R

(1) Self-Measuring System

The drum radius is found by shifting the belt with a length L=2πR₀corresponding to the circumferential length of an ideal drum andmeasuring the rotation angle θi at this time of the rotary encoder, notshown, that is directly connected to the drum. In other words, it isfound from the following equation:R=L/θi  (23)

Moreover, in the case when no rotary encoder is provided and only thereference position of the rotation is detected, the belt shiftingdistance Lb at the time of one rotation of the drum is found. In otherwords, it is found by the following equation:R=Lb/(2π)  (24)

The eccentricity ε is detected by the aforementioned two systems. In theaforementioned system in which the change in the reflection angle isdetected, for example, the detection is realized by designing theoptical system so as to use one portion of the reflected light of themain-scanning exposure beam to the drum or directing a light beam by aneccentricity-detecting light emitting element in a separate manner. Inthe case when the eccentricity is detected at a position facing thetransferring position of a toner image, the detection is made byutilizing the aforementioned means (in the case of the present system,it is of course understood that even when the exposing position or theeccentricity detection position is not located at a position facing thetransferring position of the toner image, the detection is made basedupon the same principle, although relational expressions aredifferent.).

(2) System in which Measurements are Made During the ManufacturingProcess

In the manufacturing processes of the image formation apparatus 1, R andε together with angle θ₀ information from the home position of therotary encoder, not shown, that moves in association with the rotationof the drum with ε are measured, and this information is recorded in anon-volatile memory (connected to a bus 18 of FIG. 3), not shown,provided in the image formation apparatus 1 using the tandem system, andutilized when the above-mentioned d is obtained; thus, the detection canbe realized.

4) Concerning Fluctuations in the Shifting Speed of the Conveyor Belt 7

For example, as illustrated in FIG. 9, the timing marks 21 are formed onthe belt, and the linear encoder 22 capable of detecting a timing signalin synchronism with the movement of the belt is installed; thus, theimage data is outputted as described earlier in synchronism with a clocksignal detected by this so that the image that is virtually identical isobtained although some errors in the data output timing, etc., (phaseerrors of a PLL (Phase Locked Loop), etc.) exist.

5) Output System of Image Data

In the case when there are eccentricities and deviations in the drumdiameter, an image to be generated is shifted by an amount correspondingto d in the main-scanning direction, and supposing that the pitch in thesub-scanning direction of an image formed on the belt is P, Nd=<d/P>,which has been formed into an integer by rounding off, cutting down, orraising decimals of d/P (in this specification, “<>” indicates that thenumeric value inside “<>” is obtained by forming into an integer), isset, and the address of the image data outputted in the main-scanningdirection is shifted by this value Nd=<d/P> (in this explanation,shifted in the delay direction when Nd is positive).

In this case, in order to improve the image quality, the rounding-offprocess is preferably used, and in order to obtain higher quality,interpolation information in the sub-scanning direction is added to theimage information to be scanned on the drum, and the main-scanning imagedata is selected.

(1) System in which Image Data to be Outputted onto the PhotosensitiveDrum 6 is Multiplied by N_(M) in the Sub-Scanning Direction

Here, for example, an explanation will be given of a case where N_(M)=2times.

The interpolation data is determined by estimating image data locatedbefore and after in the sub-scanning direction. The image data used atthe time of estimating is not limited to one line before and after, anda plurality of lines before and after may be used; thus, it is possibleto provide images with higher quality. Moreover, in the case wheninterpolation data in the main-scanning direction with respect to onepoint is generated, not limited to estimation made only based upon therelationship in the sub-scanning direction, estimation may be made byincluding image data in the main-scanning direction, and theinterpolation is made; thus, it is possible to further improve imagequality.

With respect to the address NC for accessing image data on the imagememory including such interpolation data, the image data address in thesub-scanning direction at the time of exposing onto an ideal drum isrepresented by “NC=2Ns+<2d/P>”, where Ns represents original dataaddress. Moreover, interpolation data in the scanning direction isincreased, and the process is carried out in the same manner. In FIG.10, the main-scanning image data is represented by a collection of datacorresponding to one dot (pixel), and data corresponding to one dot isconstituted by a plurality of bits (words) so as to representinformation (quantization level, etc.) to be recorded. In other words,this is stored in the memory on the basis of this word unit, and themain-scanning image data is also arranged so as to be taken out on theword basis.

(2) Method for Forming Interpolation Data without Increasing the StorageCapacity of the Image Memory to be Used

In the above-mentioned method the memory is multiplied by N; however,the same process is carried out without increasing this. In other words,the main-scanning image data is outputted while carrying outcalculations. That is, in order to readily carry out interpolationcalculations, the original image memory is divided, and in order tocarry out the estimation at the same time, provision is made to readilyoutput necessary image data. Moreover, in the case when the estimationis made by using the main-scanning image data, a buffer register isinstalled on the output side of the image memory so that data requiredfor interpolation is acquired from the image memory. Hereinafter, forexample, an explanation will be given of a case in which the estimationis made by using the main-scanning image data corresponding to two linesbefore and two lines after.

First, as illustrated in FIG. 11, the original image is divided intofive to form five sub-image memories, that is, sub-image memory sm1,sub-image memory sm2, . . . , sub-image memory sm5. As illustrated inFIG. 11, with respect to the main-scanning lines, in succession, 0-lineis stored in the sub-image memory sm 1, 1^(st)-line is stored in thesub-image-memory sm 2, 2^(nd)-line is stored in the sub-image memory sm3, . . . , and 5^(th)-line is stored in the sub-image memory sm1 . . . .By using a sub-image selection circuit not shown, the sub-image memorysm1 to sm5 is successively selected, and every time one cycle is madefrom the sub-image memories sm1 to sm5 in the main-scanning image data,the sub-scanning address is incremented by one. In this manner, asillustrated in FIG. 11, the main-scanning image data is stored in therespective sub-image memories sm1 to sm5.

Next, an explanation will be given of an output means for themain-scanning image data that is optically written on the drum actually.First, the sub-scanning address Ns, which has been shifted by Nd=<d/P>from the specified sub-scanning address in the case of the ideal drum,is found. Then, Ns is divided by 5, and the resulting quotient is set toa sub-scanning address register 31, and the remainder is set to asub-image selection register 32.

In order to generate interpolation data, the sub-scanning address Ns andthe main-scanning image data specified by sub-scanning addressescorresponding to before and after ±2 need to be sent to an interpolationcalculation circuit 33 at the same time. For this reason, based upon thedata set in the sub-image selection register 32 in FIG. 11, in order toselect and send the address Ns and the main-scanning image data on thesub-image memories sm 1 to sm 5 located before and after this torespective sub-scanning address decoders 34, 34, . . . respective two +1addition circuits 35, 35 and −1 subtraction circuits 36, 36 areinstalled. For example, when Ns=5, that is, when the fifth data of themain-scanning image data is selected, Ns/5=1 (remainder: 0). At thistime, 0 is set to the sub-image selection register, and 1 is set to thesub-scanning register. Therefore, with respect to the sub-image memoriessm1, sm2 and sm3, the same sub-scanning address 1 of the sub-imagememory is selected, and with respect to the sub-image memories sm4, andsm5, the sub-scanning address 0 of the sub-image memory is selected. Inother words, the main-scanning image data corresponding to before andafter two of the address Ns are simultaneously specified.

Then, among these specified main scanning image data (main scanning linedata), dot image data specified by the main scanning address register 37is outputted from the individual sub-image memories sm1 to sm5. Here,the interpolation calculation circuit 33 interpolates based uponinputted main scanning dot data; therefore, upon inputting to theinterpolation calculation circuit 33, the data is arranged so that themain scanning image data, specified by the sub-scanning address Ns, isalways located in the center. This arranging operation is carried out bya data arranging circuit 38. In the case when Ns=5, the main-scanningdot data corresponding to the sub-scanning address 5 is consequentlyplaced in the center 02, and the main scanning dot data corresponding tobefore and after data in the sub-scanning direction, are arranged,thereby providing 00, 01, 03, 04. In the conversion in the dataarranging circuit 38, based upon the outputs S0, S1, . . . , S4 of thesub-image selection register 32, the sub-image memory outputs D0, D1, .. . , D4 are arranged so as to be outputted as 00, 01, . . . , 04, bythe input/output table shown in FIG. 12.

Here, with respect to the sub-scanning address value <d/P> to be shiftedin response to the amount d shifted in the sub-scanning direction, inorder to output interpolated main-scanning image data corresponding tothe position obtained by n-dividing the pitch P, Na=<nd/P>−n<d/P> isfound, and inputted to the interpolation calculation circuit 33. Inother words, the interpolation calculation circuit 33 carries outcalculations so as to provide the main-scanning image data which looksas if it were displayed with a finer resolution of 1/n. Based upon data00, 01, . . . , 04 inputted to the interpolation calculation circuit 33,for example, an interpolation curve is drawn, and finds a valuecorresponding to the position Na×P/n, and this is outputted as themain-scanning dot data. Here, in FIG. 11, the main scanning addressdecoder 39 is a decoder used for decoding the main scanning addressinformation.

As described above, in an image formation apparatus 1 of a system inwhich exposure is carried out at a fixed position as a polygon motorsystem and exposure and scanning processes are continuously carried outwith constant intervals, it becomes possible to improve the imagequality.

6) Polygon Motor Constant Rotation System

In the case when the image formation apparatus 1 has a system in which apolygon motor rotates constantly, and exposure and scanning processesare carried out at a constant velocity in the main-scanning direction,the polygon motor is rotated constantly so that an exposing beam isallowed to scan at constant timing (time intervals) in the main-scanningdirection (in a, direction orthogonal to the drum rotation direction).The scanning timing by this polygon mirror is determined based upon anideal drum. In other words, the exposure and scanning processes arecarried out by exposing beam at a constant speed independent of the drumeccentricities and drum diameter.

The drum is rotating at a constant angular velocity ω. Therefore, in thecase of an eccentricity, if the distance from the rotation center 0 tothe exposing position E is longer than the average value of the drumradius; then, the drum peripheral velocity is greater than the averageperipheral velocity V, and if it is shorter; then the drum peripheralvelocity is smaller. The beam that is allowed to scan by the polygonmotor has a constant time interval in the beam sub-scanning direction.Therefore, as the drum peripheral velocity varies, the laser scanningpitch deviates. For example, on the drum having a longer drum radius inthe vicinity of the exposing position, the pitch interval becomes longerin the scanning.

(1) Correction of Degradation in Image Quality Due to Fluctuations inDrum Peripheral Velocity Caused by Drum Eccentricities

In order to prevent degradation in an image due to drum eccentricities,correction is made by controlling the size of dots (pixels) drawn on thedrum. In other words, the dot diameter is made greater where there arerough pitches, and the dot diameter is made smaller where there are finepitches. This change in the dot diameter is preferably made only in thesub-scanning direction in order to further improve the image quality.This correction is realized by properly controlling factors, such as thelaser optical system and irradiation laser intensity, the pulse lengthand pulse shape, in accordance with the photosensitive membercharacteristics and the charging state.

With respect to the drum peripheral velocity VL, the following equationshold, in FIG. 13. $\begin{matrix}{\begin{matrix}{{VL} = {e\;\omega\;\cos\;\beta}} \\{= {e\;{\omega\left\lbrack {1 - {\sin^{2}\beta}} \right\rbrack}^{1/2}}} \\{= {e\;{\omega\left\lbrack {1 - {{\left( {ɛ/R} \right)^{2} \cdot \cos^{2}}\theta}} \right\rbrack}^{1/2}}} \\{= {e\;{\omega\left\lbrack {1 - g} \right\rbrack}^{1/2}}}\end{matrix}{{where}\begin{matrix}\begin{matrix}{e = \left\lbrack {R^{2} + ɛ^{2} - {2R^{2}\left\{ {g - {\left( {ɛ/R} \right)\sin\;{\theta\left( {1 - g} \right)}^{1/2}}} \right\}}} \right\rbrack^{1/2}} \\{g = \left\{ {({ɛcos\theta})/R} \right\}^{2}}\end{matrix} & \;\end{matrix}}} & (25)\end{matrix}$

In this case, the above-mentioned equation is found on the assumptionthat instantaneous time changes in e and β are minimum. Therefore, thelaser output pulse is finally determined by the dot (pixel) data and theperipheral velocity.

(2) Relationship between Belt Shift and Beam Scanning on the Drum by thePolygon

In the case when the belt velocity fluctuation is zero and when there isno phase change in timing for outputting the main-scanning image datafor each sub-scanning pitch P in synchronism with the beam scanning bythe polygon and the belt shift, the image data is outputted by using theabove-mentioned means depending on the drum eccentricities and drumdiameter. However, for example, when there is a belt velocityfluctuation, this relationship is not held. Since the laser scanning bythe polygon is constantly carried out, the main-scanning image dataneeds to be outputted in synchronism with the laser main-scanning by thepolygon.

It is supposed that the main scanning image data I^(M) is specified insynchronism with the sub-scanning pitch P by the clock Ck synchronizingto the belt shift; however, the main-scanning image data I^(M) can beselected at the main-scanning image output timing Tb synchronizing tothe belt in relation to the laser beam main-scanning start time Smc. Inthis case, when there is a phase offset in the corresponding timing Tband the output timing of the main-scanning image data I^(M)synchronizing to the belt, the corresponding amount d is corrected. Inother words, in the case when the timing Tb has a delay corresponding tothe clock duty k synchronizing to the sub-scanning pitch P, the amount dis corrected by an amount of the output timing P×k, thereby forming themain-scanning image data. In some cases, k exceeds 1. If there is adelay not less than 1 pulse, k≧1 holds.

In other words, the output of the main scanning image data isinterpolated in accordance with the following equation:d=kP+π(R ₀ −R)+R sin⁻¹ {(ε/R)cos θ}  (26)2) Optical Scanning Unit System formed by Arranging a Plurality ofLight-Emitting Elements (Laser Diodes or Light-Emitting Diodes)

In this system, since the main-scanning image data can be selected atdesired time, the output of the main scanning image data is interpolatedin accordance with the following equation:d=π(R ₀ −R)+R sin⁻¹ {(ε/R)cos θ}  (27)4-2. Exposing Position Variable System

In FIG. 14, when there are an eccentricity (ε, θ) and deviations in thedrum diameter R at the exposing position E, when the main scanning imagedata is outputted at timing of an ideal drum, the transfer takes placeat a position having an advance d before the ideal transfer position,resulting in color offsets. Therefore, the exposing beam can be appliedto Cr on the drum so as to output the main-scanning image data having adelay d at crossing point E between the drum circumference and thepositive y-axis at this moment.

Thus, in order to always output at exposing timing in the same manner asthe timing at the time of the ideal drum, the exposing position iscontrolled so that the information having the delay d is located atcrossing point E between the drum circumference and the positive y-axis.Therefore, the exposure is applied to the position Cr on the drumcorresponding to a rotation angle δ from the positive y-axis in FIG. 14.After having rotated by the rotation angle δ, the exposing position Crcomes to overlap the positive y-axis. Then, after a lapse of Dt=δ/ω, animage offset corresponding d occurs in the image exposed at the exposingposition Cr, with an angle θ+δ of the eccentric position. The shift timeof the conveyor belt 7, Dd=d/v, corresponding to this amount d, isadded, and the exposure is applied to the position Cr of the angle δupon arrival to the positive y-axis, it becomes possible to eliminatethe color offset.

In other words, the color offset is eliminated when the followingrelationship is satisfied:Dt=δ/ω=Dd=d/V,In other words,δ=d/R  (28)(where, d=π(R₀−R)+R sin⁻¹ {(ε/R)cos(θ+δ)}). δ is found based upon thisrelative expression.

In other words, at the time of an angle θ of the eccentric position, theexposure is applied at the exposing position Cr that corresponds to theangle δ determined by the equation (28) before the positive y-axis. Inother words, upon outputting the main-scanning image data, theirradiation is applied at the angle δ before the positive y-axis; thus,an electrostatic latent image corresponding to the delay distance d isautomatically allowed to position on the positive y-axis after a lapseof time Dd.

The rotation center Mc of the correction-use rocking mirror is set at aposition of R₀+M on the y-axis. Here, it is supposed that the laserlight is made incident on the mirror in parallel with the x-axis. Thereflection face of the mirror-makes an angle of π/4 with respect to thex-axis in the case of the ideal drum. Suppose that the mirror rockingangle required for exposing to the exposing position Cr is θm. Moreover,supposing that the distance of the line connecting the two points of theexposing position E and the exposing position Cr is w and that the anglemade by this line with the positive y-axis is η, the following equationshold:w/sin(2θm)=(R ₀ +M−e)/sin(π−2θm−η)  (29)w/sin δ=e/sin(η−δ)  (30)−w/sin {2(β+η)}=R/sin(β+η)  (31)w/(2R)=cos(β+η)  (32)sin β=(ε/R)·cos θ  (33)β=sin⁻¹[(ε/R)·cos θ]  (34)g={(ε cos θ)/R} ²  (35)Based upon the above equations, the following equations hold:e ² =R ²+ε²−2Rε sin(β−θ)  (36)e=[R ²+ε²−2R ² {g−(ε/R)sin θ(1−g)^(1/2)}]^(1/2)  (37)Therefore,cot(2θm)=(R ₀ +M−e)/(w·sin η)−cot η  (38)Together with this equation, equations (28), (30), (32), (35) and (37)are used so as to find the mirror rocking angle θm. Based upon factorssuch as the drum radius R, the eccentricity ε and the drum rotationangle (eccentric angle) θ, the exposing beam incident angle θm iscontrolled so that the exposing position to the drum is changed and thetransfer position of the toner image is corrected. With respect to thisθm, when R and the eccentric position (ε, θ) are found, calculations arepreliminarily carried out on θ=0 to 2π by the CPU 15, the value isstored in the RAM 17, and this is inputted to a control circuit for therocking mirror section, not shown, as a reference signal in response toθ.

In this case, the main-scanning image data is outputted in synchronismwith a timing signal synchronizing to the movement of the belt. In thecase of the system in which light scanning is made by driving thepolygon mirror, the exposing beam scans on the drum in the main scanningdirection at a constant velocity. This exposing beam in themain-scanning direction is allowed to have the same signal as in thecase of the ideal drum. In other words, the image data is outputtedindependent of the eccentricities and deviations in the drum diameter.In this case, neither color offsets nor image distortion occur in animage formed on a sheet of paper.

Here, on the drum, the images in the sub-scanning direction do not haveequal pitches. In other words, since the transferring position varieswith ε cos θ, the exposing position is shifted so as to make thecorresponding correction.

The image-data output means described above can be applied not only tothe image formation apparatus 1 of the tandem type, but also to aconventional color copying machine or color printer of a single-drumtype. In other words, although the single-drum type is less susceptibleto color offsets, the above-mentioned image-data output means iseffectively applied in order to improve the image quality by preventingdegradation in the image quality such as distortion in an image due toeccentricities and deviations in the radius in the photosensitive drum.

5. Correction on Error ε_(bET0)x in Drum-to-Drum Distance D_(bET)x fromIdeal Drum-to-Drum Distance D_(bET0)x

Next, an explanation will be given of correction on errors in thedrum-to-drum installation positions of the photosensitive drum in animage formation apparatus of the tandem type. Based upon this, acorrection unit is realized.D _(bET) x=D _(bET0) x+ε _(bET0) x (x: 0, 1 , 2)  (39)

(Here, D_(bET) 0 is a distance between the photosensitive drumcorresponding to C and the photosensitive drum corresponding to M, inFIG. 1, D_(bET) 1 is a distance between the photosensitive drumcorresponding to C and the photosensitive drum corresponding to Y, andD_(bET) 2 is a distance between the photosensitive drum corresponding toC and the photosensitive drum corresponding to BK).

With respect to the main-scanning image data to the drum correspondingto M, the main-scanning image data is outputted by providing a delaycorresponding to D_(bET) 0/P sub-scanning lines from the drumcorresponding to C; with respect to the main-scanning image data to thedrum corresponding to Y, it is outputted by providing a delaycorresponding to D_(bET) 1/P sub-scanning lines from the drumcorresponding to C; and with respect to the main-scanning image data tothe drum corresponding to BK, it is outputted by providing a delaycorresponding to D_(bET) 2/P sub-scanning lines from the drumcorresponding to C, in synchronism with the above-mentioned belt.However, if D_(bET)x/P is not an integer, the corresponding correctionhas to be made.

Therefore, the drum ideal position D_(bET0)x is set to an integralmultiple of the sub-scanning pitch P. Then, supposing that thecorrection amount d to be made in response to the eccentricities anddrum diameter of each drum is dx, the correction to the aforementioned dis made by adding the error ε_(bET0)x (the increasing direction from theideal position is defined as positive) to dx, and the main-scanningimage data is then outputted.

The driving operation of the entire tandem mechanism is carried out byeither any one of the drums or any one of the belts. That is, thedriving source is single.

6. Concerning the Transfer Start Position and Timing of a Toner Image toa Sheet of Paper

Based upon this, a correction unit is realized.

In the case when a transferring process is started upon arrival of asheet of paper to the drum ideal position x=0 corresponding to C, anexposing process needs to be started prior to this. In other words, whenthe sheet of paper has reached a position with a distance πR₀ from theideal position x=0, the exposing process is started onto the drumcorresponding to C. In this case also, the corrections of dcorresponding to the eccentricities and drum diameter, and to deviationsfrom the ideal position of the drum corresponding to C, are carried out.

As described above, it is possible to form high-quality images that isless susceptible to color offsets.

7. Concerning Measurements on Deviations from the Ideal Position of theDrum (ε_(bET0)x and Measurements on Deviations of the Drum Correspondingto C)

Based upon this, a correction unit is realized.

(1) Measurements During Manufacturing Processes

Measurements are carried out during the manufacturing processes, and theresulting information is recorded in the aforementioned flash memory,not shown, of the image formation apparatus 1 using the tandem system,and this is used upon directing the above-mentioned factor d, etc.

(2) Self-Measuring System

In the polygon system, in order to allow the main-scanning image data tobe exposed onto the drum by a laser beam in synchronism with themain-scanning output timing synchronizing to the belt shift, therotation phase of the polygon motor is corrected. In the same manner asthe conventional technique, this is realized by a PLL circuit. Asillustrated in FIG. 9, reference marks 23 are put on positions on thebelt that are out of an area 7 a through which a sheet of paper passes.

In other words, in FIG. 9, each reference mark 23, which corresponds tothe leading edge position of the sheet of paper, is put on a position onthe belt that is out of an area through which a sheet of paper passes,and a leading-edge position detector (exposure start position detector)24, placed πD₀ before the ideal position of the drum corresponding to C,shows an exposure start position based on a position at which thereference mark 23 passes through this detector. Moreover, when a tonerimage is actually transferred on a sheet of paper, the paper leadingposition passage detector 25, which is placed at a position in the mainscanning direction through which a sheet of paper passes, shows theexposure start position. The main-scanning image data (without thecorrection of d at this time) is outputted so as to be transferred atthe position of the reference mark 23 on the assumption that the drum islocated at the ideal position, and as illustrated in FIG. 15, a testmark 27 actually exposed on each drum is transferred on the belt; thus,the difference from the reference mark 23 is measured so as to find adeviation from the ideal position. In other words, the timing at whichthe test mark 27 passes through the reference position error detector 26is measured by the linear encoder 22 for detecting the shift of thebelt, thereby making it possible to measure the deviation from the idealposition. In this case, a deviation d in the transferred mark positionis generated due to eccentricities of the drum and deviations in thedrum diameter; therefore, the positional deviation is calculated whilecorrecting the deviation d (a delay of d exists depending on thedefinition of the sign of d). Here, the image data of the test mark 27is not outputted at the timing in which the correction of d is firstmade; this is because proper timing has already passed due to therelationship of the eccentric position, resulting in cases in which norecording is available.

With respect to the reference mark 23, four of them, shifted in thesub-scanning direction, may be put so as to carry out the measurements.This system makes it possible to reduce the measuring elements. However,the output timing of the main scanning image data has to be shifted inthe sub-scanning direction for the corresponding shift.

8. Concerning Examples of an Operation Sequence

Based upon this, a correction unit is realized.

In FIG. 9, in each of the drums corresponding to C, M, Y and BK, adetector for detecting a reference position in the rotation angle and adetector for detecting a dislocation of the drum surface and fordetecting the subsequent eccentric position are installed, although notshown in the Figure. Moreover, although not shown, a motor for drivingthe belt is also installed.

For first example, an explanation will be given of a system in which apolygon mirror, which is driven by a polygon motor to rotate at aconstant velocity and deflects a light beam released by a laser diode,carries out a main scanning process on the drum, and an exposing(optical writing) position is fixed.

First, when power is applied to an image formation apparatus 1, the beltis driven without supplying a sheet of paper. The drum is also moved,since it is designed to integrally move with the belt, without a slip.Then, one rotation of the drum is detected by a detector for detecting areference position of the rotation angle, and the number of outputpulses of the linear encoder 22 (depending on cases, the phase of pulseintervals is also measured to improve the precision) is detected tomeasure the drum diameter. Moreover, the eccentric position is measuredbased upon the output of the detector for detecting the referenceposition of the rotation angle of one rotation of the drum and theoutput of the linear encoder 22. Since the number of output pulses ofthe linear encoder 22 corresponding to the one rotation of the drum isknown, the rotation angle is calculated. The eccentric amplitude isdetected by detecting an ac amplitude in the output waveform of thedetector of the eccentric position. The above-mentioned detections arecarried out for each of the drums. Based upon the above-mentioneddetection data, a correction value d (d=(R₀−R)+R sin⁻¹ {(ε/R)cos θ}) iscalculated for each of the drums with respect to one rotation (θ=0 to2π), and the resulting data is preliminarily stored in the RAM 17 as atable so as to be utilized later.

Next, the leading end position detector 24 placed at the end of the beltis used to detect the reference mark 23, and on the assumption that eachdrum is at an ideal position and has an ideal shape, main-scanning imagedata, which is intended to transfer a test mark 27 onto the referencemark 23, is optically written on each drum.

In the above-mentioned example, it is supposed that the main-scanningtiming phase of the polygon mirror is coincident with the sub-scanningtiming phase that is outputted in response to the shift of the belt. Inthis example, an explanation will be given of a case in which this isnot coincident. The main-scanning start timing is determined based upona pulse signal derived from the timing mark 21 on the belt detected bythe linear encoder 22; however, this is not necessarily coincident withthe main-scanning timing of the polygon mirror. Therefore, when theoutput timing of the test mark 27 fails to provide the main-scanningtiming of the polygon mirror, the main-scanning timing having a delay ofkP of the polygon motor is used to record the test mark 27. Afterdetecting an error from the reference mark 23, an amount d generated byeccentricities and deviations in the diameter and the amount kP arecorrected so that the installation error of the drum can be corrected.In this manner, the installation position of the drum, etc., and thecorrection data d for eccentricities of the drum and deviations of thediameter are found; thus, this data is used so as to output an imagethat is free from color offsets and distortions.

For a second example, an explanation will be given of the exposingposition variable system using the rocking mirror. This example alsodeals with a case in which the main scanning is carried out by a polygonmirror. Data for eccentricities and drum radius is obtained in the samemanner as described in the above-mentioned example. Here, two systemsare proposed upon recording the test mark 27. In the first system, withrespect to the angle position of the rocking mirror, it is fixed so asto have the exposing position at the origin (x=0), and the sameprocesses as described in the above-mentioned system are carried out. Inthe second system, only the correction (θm control) corresponding theeccentricities and deviations in the diameter is carried out, and thetest mark is then recorded. Other problems raised by phase difference ofthe main-scanning timing due to the polygon mirror can be solved in thesame as the first example.

Here, the following description will discuss the structure of therocking mirror. The rocking mirror is provided with an angle detectorfor detecting the rotation of this mirror, and this angle is detectedand fed back so that control is provided to obtain the target angle θm.The rocking mirror has a driving section in which a known voice coilmotor is used as a driving source, and the mirror is supported by across-shaped spring structure.

9. Technical Features

Japanese Patent Application Laid-Open No. 10-246995 discloses atechnique in which a peripheral dislocation due to eccentricities of therotation axis of the photosensitive drum is detected, and based upon thedetected dislocation, control is provided so that it becomes possible tosolve the problem due to the eccentricities in the photosensitive drum.

In contrast, in the present image formation apparatus 1, with respect tothe center point in a cross-sectional circle of the drum, the eccentricposition and drum rotation angle are detected, and based upon thedetected data, the absolute value of the amount of eccentricity, theeccentric rotation angle and the drum radius are detected; thus, controlis provided so as to correct the transfer image onto the belt or a sheetof paper.

From equation (22), the amount of correction d is represented byd=π(R₀−R)+R sin⁻¹ {(ε/R)cos θ}, and since the second term on the rightside contains the radius R and has a relationship with the rotationangle θ, this is not a simple sinusoidal relationship. Therefore, as thedemand for high resolution increases, more consideration needs to begiven to influences of the radium deviations. The above-mentionedconventional technique fails to achieve this.

In the image formation apparatus of the tandem system, conventionallydeviations in the drum diameter and drum position occur, and in thepolygon system, conventionally, when there is a fluctuation in the beltvelocity, the resulting problem is that a great color offset occurs. Theamount of data correction d is also dependent on the disk radius R;therefore, it is clear that the conventional system for detecting theperipheral dislocation fails to achieve color adjustments with highprecision because it cannot detect variations in the radius R.

With respect to the amount of detection in the eccentric dislocation,the conventional technique detects a value in proportion to e inequation (37), which is distinct from the correction method usingequation (22). These equations are shown blow. It is understood that theperipheral dislocation detection system of the conventional techniquecauses errors in the correction process.d=π(R ₀ −R)+R sin⁻¹ {(ε/R)cos θ}  (22)e=[R ²+ε²−2R ² {g−(ε/R)sin θ(1−g)^(1/2)}]^(1/2)  (37)whereg={(ε cos θ)/R} ²  (35)

Here, in the image formation apparatus 1, during the manufacturingprocesses, with respect to the relationship between the phase of theeccentric position of the drum and the position in the sub-scanningdirection of the main-scanning line transferred from the drum to a sheetof paper, the phase-adjustment is made so as to coincide all the drum;thus, in the case of the laser scanning system using the polygon mirrorwith a fixed exposing position, since the variations in the sub-scanningpitch formed on the sheet of paper are made virtually the same, itbecomes possible to further improve the image quality. However, whenthere are deviations in the drum diameter, the phase gradually comes tohave deviations as the printing or copying process is repeated. Thesedeviations can be corrected by periodical repairing maintenances.Moreover, for example, in the case when all the four drums are connectedand driven by a single motor in order to allow them to make the samerotation, there is no deviation in phases between motors. However, inthis case, all the drum diameters needs to be the same. When there aredeviations in the diameter, frictional contact occurs between the beltand the drums, resulting in fog in the image.

As described above, in the explanation of the first embodiment, theexplanation has been given of the image formation apparatus 1 providedwith the photosensitive drums 6, 6, . . . and the transfer belt 7. Inother words, in this system, the toner image formed on thephotosensitive drum 6 is directly transferred from the photosensitivedrum 6 to a sheet of paper. With respect to another system to which thepresent invention is applied, an image formation apparatus of a systemis proposed, in which toner images formed on respective photosensitivedrum 6 are transferred on a belt (intermediate transfer belt) to form acolor image, and the color image on this intermediate transfer belt istransferred on a sheet of paper by using a known means. In other words,in this case, the aforementioned direct transferring process from thephotosensitive drum 6 onto a sheet of paper may be replaced by such atransfer process onto the intermediate transfer belt.

In the explanation of the first embodiment of the present invention, theexposing position on the photosensitive drum 6 is explained as point Ein FIGS. 4 and 8, etc.; however, the above-mentioned explanation is alsotrue even when this exposing position is altered. In this case, theamount of shift of a sheet of paper or the intermediate transfer belt,which is caused by the shift of the exposing position from point E, canbe corrected. In other words, in FIG. 8, the exposing position islocated at a position having an angle z in the reversed rotationdirection of the photosensitive drum 6 from point E, a correction term,z(R₀−R), is added to equation (22). In other words, the amount of shiftof the sheet of paper or the intermediate transfer belt corresponding tothe angle z can be corrected. In this case equation (22) is formed intothe following equation (40):d=(R ₀ −R)+R sin⁻¹ {(ε/R)cos }+z(R ₀ −R)=(π+z)(R ₀ −R)+R sin⁻¹ {(ε/R)cosθ}  (40)

Of course, the above-mentioned description is not intended to limit thecontents of the present invention. For example, the present invention isalso applied to a system in which a high-speed image forming process isprovided by simultaneously forming respective electrostatic latentimages of respective colors by using a plurality of exposing beams.

The following description will discuss another embodiment of the presentinvention as a second embodiment.

FIG. 16 is a block diagram that schematically shows the structure of acopying machine 41 in accordance with the second embodiment of thepresent invention. This copying machine 41, which is a practicalembodiment of the image formation apparatus of the present invention, isconstituted by a known image reading device 42 for reading a color imagefrom a document and the image formation apparatus 1, and, based uponimage data read by the image reading device 42, the image formingprocess is carried out by the image formation apparatus 1.

Therefore, in accordance with the copying machine 41, the same functionsand effects as those of the image formation apparatus 1 of the firstembodiment are obtained.

Next, an explanation will be given of a third embodiment. The imageformation apparatus of the third embodiment is provided with aphotosensitive drum that has a round cross-section and rotates on theaxis orthogonal to the cross section, and a conveyor belt which allows asheet of paper to contact the photosensitive drum to transfer a tonerimage formed on the surface of the photosensitive drum to this paper,and transports this paper. Here, provision is made so that the contactportion between the conveyor belt or the paper and the photosensitivedrum forms an apex on the round cross section of the photosensitive drumin the conveyor belt direction.

In the embodiment of the present invention, the belt is designed as aconveyor belt for transporting paper toward the photosensitive drum.Here, with respect to the belt of the image formation apparatus of thepresent invention, another example is an intermediate transfer beltwhich allows a toner image formed on the photosensitive drum to betransferred on its surface and which also transfer this onto a sheet ofpaper.

Moreover, in the third embodiment, differences between the ideal state(designed state) of the image formation apparatus and the actual state(including eccentricities, etc. of the photosensitive drum) aredetected, and in accordance with the differences, the image formingconditions are adjusted to provide an image with high quality. In thethird embodiment, first, (1) the structure of the image formationapparatus of the third embodiment is discussed, and (2) the detection ofthe states of the image formation apparatus and (3) the adjustments ofthe image forming conditions in accordance with the states of the imageformation apparatus are then discussed.

(1) Structure of the Image Formation apparatus

FIG. 17 is a drawing that explains an essential portion of the imageformation apparatus of the third embodiment. The image formationapparatus shown in the Figure is a tandem-type image formation apparatusthat is provided with a photosensitive drum C101, a photosensitive drumM102, a photosensitive drum Y103 and a photosensitive drum K104.Moreover, the image formation apparatus is also provided with a conveyorbelt 115, a driving roller 106 on which the conveyor belt 115 iswrapped, a driven roller 105, a tension roller 114 and rollers 111, 112and 113. Below the photosensitive drum C101, the photosensitive drumM102, the photosensitive drum Y103 and the photosensitive drum K104,transferring corona chargers 107, 108, 109 and 110, which transfer tonerimages formed on the surfaces of the photosensitive drums onto sheets ofpaper, are installed.

Moreover, in the image formation apparatus of the third embodiment isprovided with an image-reading section such as a scanner, apaper-feeding section including paper-feeding cassettes, a fixingsection for fixing a toner image on a sheet of paper and apaper-discharging section. The above-mentioned structure is a well-knownstructure; therefore, the description thereof is omitted.

Each of the photosensitive drum C101, the photosensitive drum M102, thephotosensitive drum Y103 and photosensitive drum K104 has a writing unitfor writing a latent image by scanning the surface with a laser light, adeveloping device for forming a toner image by supplying toner onto thelatent image, a cleaner, a static charger, etc. These structures arealso well-known structures; therefore, drawing indicating these andexplanations thereof are omitted. Here, the developing device providedin the photosensitive drum C101 supplies cyan toner, the developingdevice provided in the photosensitive drum M102 supplies magenta toner,the developing device provided in the photosensitive drum Y103 suppliesyellow toner and the developing device provided in the photosensitivedrum K104 supplies black toner.

Each of the photosensitive drum C101, the photosensitive drum M102, thephotosensitive drum Y103 and the photosensitive drum K104 is aphotosensitive drum that has a round cross-section and rotates centeredon the axis orthogonal to the cross section. The conveyor belt 115,which is an endless conveyor belt, allows a sheet of paper, not shown,to contact each photosensitive drum so as to transfer a toner imageformed on the surface of each photosensitive drum onto the paper. Theconveyor belt 115, shown in the Figure, is shifted at a constantvelocity V in the direction of arrow.

The tension roller 114, which is designed so as to freely rotate, ispressed against the conveyor belt 115 by a spring 114 a as shown in FIG.18. Thus, the spring pressure of the spring 114 a allows the conveyorbelt 115 to contact one portion of the photosensitive drum with a propertension.

Moreover, the image formation apparatus shown in FIG. 17 is providedwith rollers 111, 112 and 113, each of which is placed between thephotosensitive drums, and no rollers for pressing the conveyor belt 115onto the photosensitive drum C101, the photosensitive drum M102, thephotosensitive drum Y103 and the photosensitive drum K104 are installed.

For this reason, positions at which the transferring processes areperformed (transfer positions) are varied due to eccentricities of thephotosensitive drums. In this case, the transfer position is varied in amanner so as to virtually coincide with an area that has the longestdistance from the rotation axis in the direction orthogonal to therotation axis within a range in which the photosensitive drum comes intocontact with the belt or the paper, and the contact portion (transferposition) between the conveyor belt or the paper and the photosensitivedrum virtually forms an apex on the round cross section of thephotosensitive drum in the conveyor belt direction.

Moreover, since there is no roller contacting the photosensitive drum,the conveyor belt 115 located at the transfer position is free frominfluences from deviations in the press-contact position due to thepress-contact roller; therefore, the rotation angular velocity of thephotosensitive drum is free from variations.

FIG. 19 is a drawing that shows a model of the photosensitive drum ofthe third embodiment. Referring to FIG. 19, the following descriptionwill discuss the fact that the image formation apparatus of the thirdembodiment is free from variations in the angular velocity of thephotosensitive drum even when the photosensitive drum haseccentricities.

As illustrated in FIG. 19, a photosensitive drum 301 having the radius Ris allowed to rotate centered on point o with an eccentricity. In FIG.19, x-axis and y-axis are given with point O serving as the origin. Whenthe center of gravity of the photosensitive drum 301 is located at G,the eccentricity is represented by the factors such as the length ε of astraight line connecting point O and point G and the angle θ made by thestraight line ε and the x-axis. Hereinafter, in the presentspecification, ε refers to “amount of eccentricity” and the positionrepresented by (ε, θ) refers to “eccentric position”.

The coordinates of the transfer position T between the photosensitivedrum 301 and the conveyor belt 302 is represented by (ε cos θ, −R+sin θ)by using the eccentric position (ε, θ). For this reason, the shiftingvelocity VTx of T in the x-direction and the shifting velocity VTy inthe y-direction are represented as follows:VTx=−ε·sin θ·ω  (41)VTy=ε·cos θ·ω  (42),where ω=dθ/dt.

Moreover, by using equation (41) and equation (42), the velocity Vs inthe rotation direction centered on point O (this direction is indicatedby a straight line S) is represented by:Vs=V·cos α−ε·sin θ·ω·cos α+ε·cos θ·ω·sin α  (43)

Here, V represents the shifting velocity of the conveyor belt 302, and αrepresents an angle that is made by the straight line S and the conveyorbelt 302, wherein the straight line S is orthogonal to a straight line rconnecting the transfer position T and point O.

Therefore, the following equation holds:ω=Vs/r=(V·cos α−ε·sin θ·ω·cos α+ε·cos θ·ω·sin α)/r  (44)In this case, the following equations hold: $\begin{matrix}\begin{matrix}{r^{2} = {R^{2} + ɛ^{2} - {2R\;{{ɛcos}\left( {{\pi/2} - \theta} \right)}}}} \\{= {R^{2} + ɛ^{2} - {{2 \cdot R \cdot ɛ \cdot \sin}\;\theta}}}\end{matrix} & (45) \\{{{{ɛ/\sin}\;\alpha} = {{r/{\sin\left( {{\pi/2} - \theta} \right)}} = {{r/\cos}\;{\theta.{Therefore}}}}},} & (46) \\{{\sin\;\alpha} = {{ɛ \cdot \cos}\;{\theta/r}}} & (47) \\{{\cos\;\alpha} = {\left( {R - {{ɛ \cdot \sin}\;\theta}} \right)/r}} & (48)\end{matrix}$By substituting equation (44) with equations (45), (47) and (48), thefollowing equation is obtained:ω={V·R−(V+ω·R)ε·sin θ+ω·ε²}/(R ²+ε²−2R·ε·sin θ)  (49).This equation (49) is transformed to:ω(R ²+ε²−2R·ε·sin θ)=V·R−(V+ω·R)ε·sin θ+ω·ε²  (49).Therefore,V=R·ω  (50).

That is, it is understood that the conveyor belt 302 is driven at theconstant velocity V so that rotation angle velocity of thephotosensitive drum 301 becomes constant without an eccentricity.

(2) Detection of States of the Image Formation Apparatus

An explanation will be given of detections of states of the imageformation apparatus, such as deviations in the transfer position due toeccentricities of the photosensitive drum, the eccentricities of thephotosensitive drum, the actual drum radius of the photosensitive drumand the operation of the writing unit.

First, an explanation is given of the detection of the transfer positionin the case when the photosensitive drum has an eccentricity. FIG. 20 isan explanatory drawing that shows a process in which the transferposition of a latent image written in the photosensitive drum 401 withan eccentricity is found. Here, in FIG. 20, the x-axis and y-axis aregiven with point O serving as the origin, which represents across-section perpendicular to the drawing paper of the rotation axis ofthe photosensitive drum 401, so that coordinates representing thephotosensitive drum 401 are given.

The photosensitive drum, shown in FIG. 20, whose cross-sectionorthogonal to the rotation axis includes the radius R, is rotatedcentered on point O with an eccentricity represented by the eccentricposition ε. Moreover, due to this eccentricity, the center of gravity ofthe photosensitive drum 401 is varied in a manner so as to shift on thecircumference of the circle having the radius ε.

In the photosensitive drum 401, after a latent image has been written,the latent image, thus written, is formed into a toner image by thedeveloping device, and this is transferred on a sheet of paper. In thecase when the photosensitive drum 401 has no eccentricity, the latentimage is written at upper point (0, R) that crosses the y-axis withinthe photosensitive drum 401. Then, after a lapse of a predeterminedtime, the photosensitive drum 401 has been π-rotated so that the tonerimage is transferred on a sheet of paper at lower point (0, −R) thatcrosses the y-axis.

However, when its center of gravity is located at G₂, the latent imageis written at a portion represented by E in the photosensitive drum 401.The latent image, thus written, is developed by the developer to form atoner image. When the photosensitive drum 401 having the eccentricity isΘt-rotated to have its gravity at G₁, it is transferred onto a sheet ofpaper, not shown, at transfer position T₁. At this time, the transferposition T₁ has an offset corresponding to −s (s: referred to as “offsetamount”) in the x direction from the transfer position (0, −R) withoutany eccentricity in the photosensitive drum 401.

The offset amount s is found as follows:

The rotation angle Θt in which the photosensitive drum 401 is rotatedfrom the latent-image writing position to the transfer position isrepresented as follows by using an angle β represented by the angleG₂EO:Θt=π−β  (51).Since the following equation holds:sin β=(ε/R)cos θ  (52),the following equation is given:Θt=π−sin⁻¹[(ε/R)cos θ]  (53)Since s=ε·cos(θ−β), the following equation is obtained:s=ε·cos θ(ε/R)[{(R/ε)²−cos² θ}^(1/2)+sin θ]  (54)Therefore, even in the case when the photosensitive drum 401 has aneccentricity, if the greatest eccentric amount ε and the angle θ (FIG.20) made by the greatest eccentricity at the moment (at which exposureis made) when the latent image is written are found, it is possible tojudge the rotation angle Θt in which the photosensitive drum 401 isrotated from the latent-image writing position to the transfer positionand the offset of the transfer position in the x-axis direction. Asdescribed above, when there is an offset in the transfer position, thetransfer position of the toner image on the conveyor belt 402 is alsosubjected to an offset. The offset amount d of the positional offset ofthe toner image on the conveyor belt is found as follows:

The toner image, which has been formed based upon the latent imagewritten on the photosensitive drum 401, is transferred after thephotosensitive drum 401 has been rotated by Θt from the writing processof the latent image. For this reason, supposing that the angularvelocity of the rotation of the photosensitive drum 401 is ω, the time τtaken from the writing process of the latent image to the transfer isrepresented as follows:Θt/ω=τ  (55).

Moreover, in the case when a photosensitive drum (ideal drum) having theradius R₀ as designed is rotated at an angular velocity ω₀, the time τ₀taken from the writing process of the latent image to the transfer isrepresented by:π/ω₀=τ₀  (56)In other words, between the photosensitive drum 401 having theeccentricity and the ideal drum, there is a time difference of τ₀−τ fromthe writing process to the transfer. For this reason, with respect tothe photosensitive drum 401 having deviations in the radius andeccentricities, the offset amount d between the transfer position on thetransfer belt 402 thereof and the transfer position on the transfer belt402 of the ideal drum is represented by: $\begin{matrix}\begin{matrix}{d = {V\left( {\tau_{0} - \tau} \right)}} \\{= {V\left( {{\pi/\omega_{0}} - {\Theta\;{t/\omega}}} \right)}} \\{= {{\pi\left( {R_{0} - R} \right)} + {R\;\sin^{- 1}\left\{ {\left( {ɛ/R} \right)\cos\;\theta} \right\}}}}\end{matrix} & (57)\end{matrix}$

The offset amount d indicates that, in the case when a latent image isformed by generating image data on the assumption of the ideal drum, ifthere are eccentricities or deviations in the radius, the image istransferred at position having an offset of d, resulting in a coloroffset. Therefore, an image that is to be transferred at the positionhaving the offset of d is preliminarily generated and the correspondinglatent image is formed, the image corresponding to the transfer positioncan be formed with the result that no color offset is generated.

As described above, by operating the writing unit while taking intoconsideration the offset amount d thus found, the image formationapparatus of the third embodiment makes it possible to transfer a tonerimage at the same position as the toner image transferred by the idealdrum, even when the photosensitive drum has eccentricities or when thephotosensitive drum has a radius different from the radius of the idealdrum.

In order to obtain the offset amount d, it is necessary to detect theradius R of the actual photosensitive drum, (ε, θ) indicating theeccentric position and the drum rotation angle Θt. Next, an explanationwill be given of the arrangement of an image formation apparatus fordetecting the radius R, the eccentric position (ε, θ) and the rotationangle Θt.

FIG. 21 is a drawing that explains an arrangement in which the imageformation apparatus of the third embodiment detects the radius R of thephotosensitive drum, the eccentric position (ε, θ) and the rotationangle Θt of its own apparatus. Here, in FIG. 21, those devices havingthe same functions as those shown in FIG. 17 are represented by the samereference numerals, and the description thereof is omitted.

The structure shown in the Figure is provided with a photosensitive drumC101, a photosensitive drum M102, a photosensitive drum Y103 and aphotosensitive drum K104, and a conveyor belt 115. Moreover, thephotosensitive drum C101, the photosensitive drum M102, thephotosensitive drum Y103 and the photosensitive drum K104 arerespectively provided with rotation angle detecting encoders 501, 502,503 and 504 for detecting rotation angles, as well as eccentricitydetectors 521, 522, 523 and 524 for detecting eccentricities.

Moreover, the conveyor belt 115 is provided with a paper-passage area512 on which sheets of paper are transported to pass. Inside thepaper-passage area 512, a paper-passage detector 508 for detectingpassage of sheets of paper is installed. Outside the paper-passage area512, a timing mark 510 for detecting the shift amount of the conveyorbelt 115 and a reference mark 511 for indicating the leading edge of theconveyor belt 115 are formed. Furthermore, above the conveyor belt 115,a linear encoder 507 for detecting the timing mark 510 and forgenerating a pulse each time the timing mark is detected, a leading edgeposition detector 509 for detecting the reference mark and a referenceposition error detector 506 for detecting an offset amount betweenpositions of the toner image and the reference mark, which will bedescribed later, are installed.

The rotation angle detecting encoders 501, 502, 503 and 504 of the thirdembodiment are encoders that can detect an absolute value of therotation angle of the photosensitive drum, and generates a pulse eachtime the photosensitive drum makes a rotation with a predeterminedangle. The rotation angle detecting encoders 501, 502, 503 and 504function as writing start position detecting means for detecting aposition from which the writing operation is started to each of thephotosensitive drums.

Moreover, each of the eccentricity detectors 521, 522, 523 and 524 ofthe third embodiment is provided with a light-emitting element forapplying a light beam on the outer surface of each photosensitive drum,a light-receiving element for receiving the light beam reflected fromthe outer surface of the photosensitive drum, and an optical system fordetecting a change in the quantity of received light of thelight-receiving element (for example, two-division photodiode) when theouter surface of the photosensitive drum is dislocated due toeccentricities. For such an eccentricity detector, for example, a focuserror detection system used in the field of optical disks can beadopted.

In the case when the focus error detection system is used as theeccentricity detector for detecting the eccentricity, a photocurrent,which corresponds to the variation in the distance between the eccentricdetector and the photosensitive drum outer surface, is allowed to flowthrough the light-receiving element. By taking out the photocurrent fromthe light-receiving element as an electrical signal, it is possible toobtain a waveform curve that varies with a constant cycle and amplitude.Based upon the amplitude and cycle, the eccentric position (ε, θ) isobtained.

FIG. 22 is a drawing that shows a reference mark 511 and toner images601C, 601M, 601Y and 601K serving as reference toner images that thephotosensitive drum C101, photosensitive drum M102, photosensitive drumY103 and photosensitive drum K104 have formed on the conveyor belt 115based on the reference mark 511. A writing unit, not shown, writes alatent image (reference latent image) on the photosensitive drum C101 onthe assumption that the photosensitive drum C101 is an ideal drum. Thereference latent image is developed by a developing device not shown,and transferred on the conveyor belt 115 as a toner image 601C. Thewriting and transferring processes of the reference latent image arecarried out in synchronized timing so that the toner image 601C istransferred while being coincident with the reference mark 511.

Moreover, in the same manner, the writing unit of the photosensitivedrum M102, the writing unit of the photosensitive drum Y103 and thewriting unit of the photosensitive drum K104 also transfer toner image601M, toner image 601Y and toner image 601K on the transfer belt 115 ina manner so as to be coincident with the reference mark 511. As aresult, as illustrated in FIG. 22, with respect to each of thephotosensitive drums, patterns, which indicate differences between theideal writing process and transfer timing and the actual writing-processand transfer timing, are formed. In this case, in the third embodiment,the pattern shown in FIG. 22 is directly formed on the conveyor belt 115without transporting a sheet of paper.

FIG. 23 is a block diagram that explains the construction that controlsthe structure shown in FIG. 21. The construction shown in the Figureincludes: a reference position error detector 506 described in FIG. 21,a linear encoder 507, rotation angle detecting encoders 501, 502, 503and 504, eccentricity detectors 521, 522, 523 and 524, and a controlsection 701 to which information detected by the rotation angledetecting encoders 501, 502, 503 and 504 and the eccentricity detectors521, 522, 523 and 524 is inputted, and which calculates the radius R andthe eccentric position (ε, θ) based upon the information, and controlsthe writing unit control sections 702, 703, 704 and 705 based upon thecalculated values.

In the construction shown in FIG. 23, the control section 701 functionsas a timing adjusting means which adjusts generation timing of data tobe written in each of the photosensitive drums, based upon actualmeasurement values of the eccentricity detectors 521, 522, 523 and 524,the reference position error detector 506, the linear encoder 507 andthe rotation angle detecting encoders 501, 502, 503 and 504.

A memory 706 for storing the calculated values is connected to thecontrol section 701. Moreover, operation signals for operating the imageformation apparatus are inputted thereto. Four writing unit controlsections 702, 703, 704 and 705 are installed in association with therespective writing units installed in the photosensitive drum C101,photosensitive drum M102, photosensitive drum Y103 and photosensitivedrum K104.

The construction shown in FIGS. 21 and 23 detects the radius R, theeccentric position (ε, θ) and the rotation angle Θt through operationsas described below. In other words, when an image forming process isstarted, the control device, not shown, which controls the entire imageformation apparatus, drives the driving roller 106 so as to shift theconveyor belt 115. At this time, the linear encoder 507 detects thetiming mark 510 of the conveyor belt 115. Based upon this signal, thecontrol section 701 detects the shift amount of the conveyor belt 115,and thus detects that the conveyor belt 115 has been shifted thecircumferential length L (2πR₀) of the ideal drum.

Here, the rotation angle detecting encoders 501, 502, 503 and 504respectively input the rotation angles of the photosensitive drum C101,photosensitive drum M102, photosensitive drum Y103 and photosensitivedrum K104 to the control section 701. Based upon the rotation angle θiof each photosensitive drum at which the conveyor belt 115 has beenshifted length L, the control section 701 finds the actual radius R ofeach photosensitive drum from the following equation:R=L/θi  (58).

Though the above-mentioned operations, the actual radiation R of each ofthe photosensitive drum C101, photosensitive drum M102, photosensitivedrum Y103 and photosensitive drum K104 is calculated. The calculatedradius R is outputted from the control section 701 to the memory 703,and stored therein. Moreover, the rotation detecting encoders 501, 502,503 and 504 input the rotation angles of the photosensitive drum C101,photosensitive drum M102, photosensitive drum Y103 and photosensitivedrum K104 to the control section 701. Then, the eccentricity detectors521, 522, 523 and 524 input information related to eccentricities of thephotosensitive drum C101, photosensitive drum M102, photosensitive drumY103 and photosensitive drum K104 into the control section 701 aseccentricity signals. The control section 701 is allowed to obtaininformation related to the eccentric position (ε, θ) from theinformation related to the rotation angles of the photosensitive drumsand the eccentricity signals.

(3) The Adjustments of the Image Forming Conditions in Accordance withthe States of the Image Formation Apparatus

Next, an explanation will be given of adjustments of the image formingconditions in the image formation apparatus in accordance with thecalculated radium R (ε, θ). Here, in this case, θ is rotation angleinformation corresponding to the greatest eccentric amount ε at theinstantaneous time of formation of the latent image. The control section701 substitutes R and ε thus calculated into equation (57) so that theoffset amount d during a period (θ=0 to 2π) of one rotation of eachphotosensitive drum. The offset amount d of the transfer positions on asheet of paper, thus calculated, is stored as a reference table in thememory 703. Then, in accordance with the rotation angles inputted fromthe rotation angle detecting encoders 501, 502, 503 and 504, the controlsection 701 refers the offset amount d of the corresponding rotationangle so as to control the writing unit control sections 702, 703, 704and 705.

In the control process, each of the writing unit control sections 702,703, 704 and 705 adjusts the data writing timing of the latent imagewith respect to each of the photosensitive drums in response to theoffset amount d so that the toner image is transferred onto a constantposition on the conveyor belt 115 independent of the eccentricities ofeach photosensitive drum or the deviations in the radius of eachphotosensitive drum. Consequently, toner images of respective colors,cyan, magenta, yellow and black, transferred onto the respectivephotosensitive drums, are superposed on a sheet of paper without coloroffsets, and properly transferred, independent of the eccentricities ofeach photosensitive drum or the deviations in the radius of eachphotosensitive drum.

Moreover, in the construction shown in FIG. 23, the reference positionerror detector 506 detects the offset amount between the reference mark511, shown in FIG. 22, and the toner image 601C, toner image 601M, tonerimage 601Y and toner image 601K. The offset amount, thus detected, isinputted to the control section 701. Upon formation of the toner image601C (601M, 601Y, 601K), when this has been subjected to the correctionof the offset amount d, the writing unit control sections 702, 703, 704and 705 are controlled by the control section 701 so as to adjust thewriting timing of the writing unit in response to the inputted offsetamount.

Moreover, when the control section 701 forms the toner image 601C (601M,601Y, 601K) without carrying out the correction of the offset amount d,calculation processes may be carried out based upon the offset amountdetected by the reference position error detector 506 so as to correctthe offset amount d; thus, control may be provided so as to correctdeviations in the installation positions of the photosensitive drums.Through this control operation, the image formation apparatus of thethird embodiment makes it possible to cancel offsets in the transferpositions even where there are deviations in the installation positionsin the respective photosensitive drums.

FIG. 24, which explains the control method of the image formationapparatus carried out in the image formation apparatus of the thirdembodiment, is a flow chart that particularly shows the detection of thestate of the image formation apparatus and the adjustments of the imageforming conditions. A sequence of the processes shown in the flow chartof the Figure is started when the control section 701 receives aninstruction for a latent image forming control by an operation signal(step S801), and until the instruction for the latent image formingcontrol has been given (step 801: No), the sequence is in the stand-bystate.

When the sequence is started, the control section 701 detects the shiftamount of the conveyor belt 115 by the linear encoder 507 in a statewhere the conveyor belt is being driven by another control section notshown, thereby calculating the radius R of each photosensitive drum(step S802). Moreover, the eccentricity detectors 521, 522, 523 and 524and the rotation angle detecting encoders 501, 502, 503 and 504 are usedfor detecting the eccentric position (ε, θ) (step S803) so that theoffset amount d is calculated in combination with the radius R found instep S802. At this time, with respect to the offset amount d, all thevalues taken as θ varies from 0 to 2π in equation (57) are calculated.The offset amounts d thus calculated are stored in the memory 703 as areference table (step S804).

Moreover, the control section 701 detects the offset amounts between thereference mark 509 and the toner images 601C, 601M, 601Y and 601K (stepS805). Then, the offset amount d corresponding to the rotation angle ofeach photosensitive drum at the instantaneous time of the formation ofthe latent image is read out (step S806), and by taking intoconsideration both the offset amount d and the offset amount between thereference mark and the toner image, control signals for controlling thewriting unit control sections 702, 703, 704 and 705 are generated (stepS807). The control signals generated in step S807 are outputted to thewriting unit control sections 702, 703, 704 and 705 (step S808).

Next, the control section 701 makes a judgment as to whether or not thelatent image formation has been completed (step S809). When the judgmentat step S809 shows that the latent image formation has not beencompleted (step S809: No), the sequence again returns to step S806 toread out the offset amount d, thereby generating a control signal tooutput to the writing unit control sections 702, 703, 704 and 705.

In contrast, when the judgment of step S809 shows that the imageformation has been completed (step S809: Yes), a judgment is made as towhether or not a latent image formation for the next page is carried out(step S810). When this judgment shows that there is no latent imageformation for the next page (step S810: No), the sequence of processesis completed. Here, the offset amount d is reset when the mechanicalstate of the image formation apparatus has been changed by, for example,maintenance of the image formation apparatus or a mechanical adjustmentthereof, or it is reset periodically for every predetermined period.Moreover, the judgment at step S810 shows that there is a latent-imageformation for the next page (step S810: Yes), the sequence proceeds tostep S811.

Next, a judgment is made as to whether or not an instruction for alatent image formation is given (step S811) Then, upon receipt of theinstruction for the latent-image formation (step S811: Yes), d is readfrom the memory 703 so that the writing unit control sections 702, 703,704 and 705 are controlled. Moreover, during a period in which noinstruction for the latent-image formation is given, the control section701 is in the stand-by state to be ready for the instruction (step S811:No).

The above-mentioned controlling method of the image formation apparatusis realized by executing a preliminarily prepared program by using acontroller, not shown. Alternatively, this program may be recorded on arecording medium that is read by a computer, such as a hard disk, afloppy disk, a CD-ROM, an MD and a DVD, and this is read by thecomputer, and transferred to the above-mentioned controller so as to beexecuted therein. Moreover, this program may be distributed through therecording medium or a network, such as the Internet, serving as atransmitting medium.

Here, the present invention is not intended to be limited by the thirdembodiment. For example, in the image formation apparatus in the presentinvention, in place of the rotation angle detecting encoder, a pulsegenerator for generating one pulse each time the photosensitive drummakes one rotation (reference position detection unit for detecting thereference position of the rotation angle of the photosensitive drum) maybe used for detecting the rotation angle of the photosensitive drum. Inthe case when the pulse generator is used for detecting the rotationangle, in synchronism with the pulse generation cycle (period in whichthe photosensitive drum makes one rotation) of the pulse generator, apulse generated by the linear encoder 507 is counted so that therotation angle in which the photosensitive drum rotate while the linearencoder 507 generates one pulse is detected.

Moreover, in the case when the radius R of the photosensitive drum isfound by using such a pulse generator, the shift amount (Lb) of theconveyor belt 115 while the photosensitive drum makes one rotation isdetected by the linear encoder 507. Then, the radius R of thephotosensitive drum is calculated by the following equation:R=Lb/2π  (59).

Moreover, the control section 701 in the image formation apparatus ofthe present invention may have an arrangement in which a rotation angledetecting encoder is placed at any one portion of the photosensitivedrum or the roller axis supporting the conveyor belt of thephotosensitive drum, with the other photosensitive drums having norotation angle detecting encoder being provided with the above-mentionedpulse generators; thus, it becomes possible to detect (introduce) theoffset amount d in the transfer position on a sheet of paper on theconveyor belt 115 of the photosensitive drum.

Moreover, the radius R and the eccentric position (ε, θ) may be measuredon the factory side prior to the shipment of the image formationapparatus, and these data may be stored in a flash memory installedinside the image formation apparatus.

Next, an explanation will be given of a fourth embodiment of an imageformation apparatus in accordance with the present invention. The imageformation apparatus of the fourth embodiment of the present inventionhas an arrangement in which the image formation apparatus described inthe third embodiment is further provided with a motor which rotates thephotosensitive drum K in synchronism with a pulse generated by therotation angle detecting encoder every time the photosensitive drumrotates with a predetermined rotation angle, in order to reduce a slipbetween the photosensitive drum and the conveyor belt.

FIG. 25 is a drawing that explains an essential portion of the imageformation apparatus of the fourth embodiment. In the same manner as theimage formation apparatus of the third embodiment, the image formationapparatus shown in the Figure is a tandem-type image formation apparatusthat is provided with a photosensitive drum C901, a photosensitive drumM902, a photosensitive drum Y903 and a photosensitive drum K904.Moreover, the image formation apparatus is provided with a conveyor belt915, a roller 905 on which the conveyor belt 915 is passed, a roller906, a tension roller 914, rollers 911, 912 and 913. Above the conveyorbelt 915, a leading position detector 909 for detecting the leading edgeof the conveyor belt 915 is installed so as to detect a reference mark,not shown, formed on the conveyor belt 915.

Here, the image formation apparatus of the fourth embodiment is alsoprovided with an image-reading section such as a scanner, apaper-feeding section including paper-feeding cassettes, a fixingsection for fixing a toner image on a sheet of paper, apaper-discharging section and a corona charger. Moreover, each of thephotosensitive drum C901, the photosensitive drum M902, thephotosensitive drum Y903 and photosensitive drum K904 has a writing unitfor writing a latent image by scanning the surface with a laser light, adeveloping device for forming a toner image by supplying toner onto thelatent image, a cleaner, a static charger, etc. These structures arealso well-known structures; therefore, drawing indicating these andexplanations thereof are omitted.

Here, the developing device provided in the photosensitive drum C901supplies cyan toner, the developing device provided in thephotosensitive drum M902 supplies magenta toner, the developing deviceprovided in the photosensitive drum Y903 supplies yellow toner and thedeveloping device provided in the photosensitive drum K904 suppliesblack toner.

In the image formation apparatus shown in FIG. 25, the photosensitivedrum C901, the photosensitive drum M902 and the photosensitive drum Y903are respectively provided with rotation angle reference positiondetectors 931, 932 and 933. Moreover, the photosensitive drum K904 isprovided with a rotation angle detecting encoder 908. The rotation anglereference position detectors 931, 932 and 933 are provided with pulsedetectors each of which generates a pulse each time the photosensitivedrum rotates once, and an encoder for detecting the rotation angle ofthe photosensitive drum as an absolute value is used as the rotationangle detecting encoder 908.

Moreover, the photosensitive drum C901, the photosensitive drum M902,the photosensitive drum Y903, the photosensitive drum K904 and theroller 906 are respectively provided with load variation correctingmotors 921, 922, 923, 924 and 926, and the photosensitive drum K904 hasa motor 907 that is directly connected to the rotary axis.

The motor 907 is a motor that drives the conveyor belt 915 to rotate. Inother words, the motor 907 rotates to allow the photosensitive drum K904to rotate so that the conveyor belt 915 contacting the photosensitivedrum K904 is shifted. In association with the rotation of the conveyorbelt 915, the rollers 905, 906, 911, 912, 913 and the tension roller 914are allowed to rotate. Moreover, the load variation correcting motors921, 922, 923, 924 and 926 detect variations in the loads imposed on thephotosensitive drum C901, the photosensitive drum M902, thephotosensitive drum Y903, the photosensitive drum K904 and the roller906, and the detected variations are subjected to the motor torque so asto reduce the load variations.

Between the photosensitive drum and the conveyor belt (or anintermediate transfer belt), between the photosensitive drum and paper,as well as between the paper and the conveyor belt, if a force in thebelt shifting direction that is greater than a frictional force isapplied to the place where the frictional force is generated, slippingoccurs. The same is true for the contact portion between the belt andthe belt driving roller. Thus, the load variation correcting controlsystem is installed so as to prevent the force greater than thefrictional force from being generated.

Here, the loads variations which are suppressed by the load variationcorrecting motors 921, 922, 923, 924 and 926 are loads generated by acleaner, etc., located on the outer surface of the photosensitive drumor loads generated by a cleaner, etc., located on the periphery of theconveyor belt. These loads tend to vary periodically. Here, with respectto the control of the image formation apparatus by the rotation angledetecting encoder 908 and the motor 907 and the operations of the loadvariation correcting motors 921, 922, 923, 924 and 926, the descriptionthereof will be given later.

As illustrated in FIG. 26, each load variation correcting motor (theload variation correcting motor 924, in the Figure) can be attached tothe photosensitive drum (the photosensitive drum K904, in the Figure) ata portion outside an area in which a writing operation is performedthrough a comparatively small motor 1001. Thus, the application of theload variation correcting motor 924 makes it possible to increase themotor efficiency and to reduce power consumption, in comparison with thedirect connection of the motor to the photosensitive drum, so that it ispossible to use a smaller motor, and consequently to reduce the size ofthe image formation apparatus described in the fourth embodiment.

Moreover, in the image formation apparatus in the fourth embodiment,pulses generated by the rotation angle detecting encoder 908 aredetected, and the measured values of the pulses are compared with theshift amount of the conveyor belt 915 detected by the leading positiondetector 909. Based upon these comparisons, it is possible to detect theshift amount of the conveyor belt 915 that is made while thephotosensitive drum rotates once in the image formation apparatus of thefourth embodiment. Alternatively, it is possible to detect the number ofpulses generated by the rotation angle detecting encoder 908 while theconveyor belt 915 rotates once.

Furthermore, in the image formation apparatus of the fourth embodiment,which relates to the image formation apparatus of the third embodimentthat uses a driving roller to shift the conveyor belt, the shift amountof the conveyor belt that is made while the photosensitive drum rotatesonce is preliminarily stored. Alternatively, the number of pulsesgenerated by the rotation angle detecting encoder 908 while the conveyorbelt 915 rotates once is preliminarily stored. Then, the shift amountthus stored is compared with the detected shift amount of the conveyorbelt 915, and if the difference exceeds a permissible range, makes ajudgment that the image formation apparatus is in an abnormal state.

FIG. 27 is a block diagram that explains the controlling operation ofthe image formation apparatus carried out by the rotation angledetecting encoder 908 and the motor 907. The construction, shown in FIG.27, is provided with: a photosensitive drum K904, a motor 907 forrotating the photosensitive drum K904, the rotation angle detectingencoder 908 for detecting the rotation angle of the photosensitive drum904, a control circuit 1100 for controlling the motor 907 based upon asignal detected by the rotation angle detecting encoder 908, a poweramplifier 1104, a phase compensating device 1105, an f-V converter 1106and an encoder pulse detector 1107.

Moreover, the control circuit 1100 is provided with a phase comparator1101, a charge pump 1102, and LPF (Low Pass Filter) 1103.

For example, the control section of the motor 907, not shown, prepares apulse that is equal in frequency to a pulse that has been outputted bythe rotation angle detecting encoder 908 when the photosensitive drumK904 has reached a target rotation velocity ω, and outputs this pulse tothe phase comparator 1101 as a reference pulse S1. The phase comparator1101 receives a pulse signal S4 formed based upon a pulse that therotation angle detecting encoder 908 has actually outputted, andcompares this with the reference pulse S1 to calculate a phasedifference between them. The calculated value is converted into avoltage signal that is represented in an analog format after passingthrough the charge pump 1102 and the LPF 1103, and further inputted tothe power amplifier 1104. The above-mentioned process is a well-knownprocess, that is, a so-called PLL (Phase Locked Loop) process.

Here, the pulse, generated by the rotation angle detecting encoder 908,is outputted to the f-V converter 1106 through the encoder pulsedetector 1107. The f-V converter 1106 converts the pulse to a voltagesignal to generate a voltage signal S3 that is proportional to theangular velocity of the photosensitive drum K904. This voltage signal S3is fed back to the power amplifier 1104 through the phase compensatingdevice 1105 so that the velocity controlling characteristic of thephotosensitive drum is improved.

Moreover, since the voltage signal S3 is directly proportional to theangular velocity of the photosensitive drum K904, this is outputted toother constructions as the signal indicating the rotation velocity ofthe photosensitive drum K904 so as to be used for velocity control inother systems constituted by the photosensitive drum and the conveyorbelt.

Moreover, in the case when the photosensitive drum K904 is not providedwith the load variation correcting motor 924, variations in the load(magnitude, timing) imposed on the photosensitive drum K904 arepreliminarily found, and these variations in the load are appliedthereto from the external device as a feed forward signal S2. Such afeed forward control makes it possible to improve the velocity controlcharacteristic of the photosensitive drum even when no load variationcorrecting motor 924 is installed.

Here, the timing and magnitude of load variations imposed on thephotosensitive drum in an apparatus forming electronic photographs (aprinter or a copying machine) are preliminarily known. Thus, theabove-mentioned feed forward control is available.

FIG. 28 is a block diagram that explains the construction forcontrolling the load variation correcting motor. The construction, shownin the Figure, is provided with a drum-C load variation correctingcontrol system 1200, for correcting the load variations in thephotosensitive drum C901, a feed forward signal generator 1210, a drum-Mload variation correcting control system 1206 for correcting the loadvariations in the photosensitive drum M902, a drum-Y load variationcorrecting control system 1207 for correcting the load variations in thephotosensitive drum Y903, a drum-BK load variation correcting controlsystem 1209 for correcting the load variations in the photosensitivedrum K904 and a conveyor belt load variation correcting control system1208 for correcting the load variations in the conveyor belt 915.

Moreover, the drum-C load variation correcting control system 1200 isprovided with a current supply type power amplifier 1201, a phasecompensating device 1202, a load variation correcting motor 921, arotation motor reverse electromotive force detector 1204 and acomparator 1205 for use in the rotation motor reverse electromotiveforce detection.

The above-mentioned construction makes it possible to cancel the loadvariations in the photosensitive drum by using the torque of the loadvariation correcting motor 921, so as to minimize the influences thatare given by the load variations in the photosensitive drum to the otherphotosensitive drums through the conveyor belt.

The feed forward signal generator 1210 inputs signals for generatingtorques for canceling the known load variations (magnitude, timing)improved on the photosensitive drum C901, photosensitive drum M902,photosensitive drum Y903, photosensitive drum K904 and the conveyer belt915, that is, feed forward signals, into the corresponding controlsystems, that is, the drum-C load variation correcting control system1200, the drum-M load variation correcting control system 1206, thedrum-Y load variation correcting control system 1207, the drum-BK loadvariation correcting control system 1209 and the conveyor belt loadvariation correcting control system 1208.

Moreover, the voltage signal S3 is respectively inputted to the drum-Cload variation correcting control system 1200, the drum-M load variationcorrecting control system 1206, the drum-Y load variation correctingcontrol system 1207, the drum-BK load variation correcting controlsystem 1209 and the conveyor belt load variation correcting controlsystem 1208. The signal S3 is a signal for determining the rotationvelocity of the load variation correcting motor. In this case, withrespect to the conveyor belt load variation correcting control system,the signal S3 is converted to a value so as to allow the conveyor belt915 and the photosensitive drum to rotate integrally, thereby forming areference signal that determines the rotation of the motor, and thissignal is inputted thereto.

Moreover, the reference signal for each of the photosensitive drums,inputted to the load variation correcting control system, may be formedinto a reference signal for determining the rotation of the motor, whichis obtained by taking into consideration the deviations in the radius ofthe photosensitive drum to make a correction. Furthermore, the drum-Cload variation correcting control system 1200 of FIG. 28 detects areverse electromotive force generated in proportion to the rotationvelocity of the load variation correcting motor 921, and compares thiswith the voltage signal S3 so as to control the velocity. In the fourthembodiment, the other load variation correcting control systems alsohave the same circuit construction. The load variation correctingcontrol system provides control so that even when there is a loadvariation in the speed determined by the reference signal fordetermining the rotation of the load variation correcting motor, therotation is maintained. In other words, consequently, the amount of theload variations that is transmitted to the other driving systems isreduced, with the result that it is possible to reduce slipping thatoccurs between the photosensitive drum and the conveyor belt or a sheetof paper.

In accordance with the above-mentioned operations, the load variationcorrecting motors 921, 922, 923, 924 and 926 are controlled in responseto the rotation angular velocity of the motor 907 for driving thephotosensitive drum 904. For this reason, the image formation apparatusof the third embodiment is provided with a construction in which even atthe time of activation, the photosensitive drum and the conveyor belt915 are less susceptible to slipping (in which only either of thephotosensitive drum and the conveyor belt 915 is shifted). Here, thereverse electromotive force generated in the load variation correctingmotor 921 can be detected by subtracting an inner resistance value fromthe voltage applied to the terminal. Moreover, in order to improve thecontrolling characteristics of the construction shown in FIG. 28, thecurrent supply type power amplifier 1201 is used as the power amplifierand the phase compensating device 1202 is installed.

For another construction example, the signal for determining therotation angular velocity of the load variation correcting motor may begenerated by a controller, not shown. Although other control modes suchas starting and stopping modes are not described in the thirdembodiment, these can be achieved by using the conventional technique.

As described above, in accordance with the image formation apparatus ofthe third embodiment, the driving system including the photosensitivedrum and the conveyor belt 915 is velocity-controlled by the motor 907,and the load variations imposed on the other photosensitive drums andthe conveyor belt 915 that are driven in accordance with thephotosensitive drum K904 rotated by the motor 907 are corrected by theindividual control systems.

Next, an explanation will be given of another structural example of theload variation correcting motor of the fourth embodiment. FIG. 29 is adrawing that shows an example of the load variation correcting motor inwhich: a roller 134 is installed on the rotary axis of thephotosensitive drum K904, and the roller 131, which is directly rotatedby the load variation correcting motor 132, is allowed to contact theroller 134 so as to drive this. Here, reference number 133 represents anencoder. In the construction shown in FIG. 29, it is possible to improvethe degree of freedom for the structure for transmitting the drivingforce of the load variation correcting motor 132 to the photosensitivedrum K904.

FIG. 30 is a drawing that shows an example of the load variationcorrecting motor that supports a roller 141 rotated a the load variationcorrecting motor 144 by using a spring 142. In accordance with theconstruction shown in FIG. 30, it is possible to sufficiently transmitthe driving force of the load variation correcting motor 144 to thephotosensitive drum K904.

FIG. 31 is a structural example of the load variation correcting motorin which a disc-shaped transmitting member 151 directly connected to aphotosensitive drum 153 and a roller 152 for stably transmitting thedriving force of a driving source 154 are used. In the contact portionof the transmitting member 151 and the roller 152, the transmittingmember 151 side has a flat face and the roller 152 has a tapered shapeso as to increase the contact portion, thereby providing a stabletransmitting characteristic (characteristic for preventing slipping).The rotary axis of the driving source 154 is not in parallel with thesurface orthogonal to the rotary axis of the transmitting member 151.

Moreover, in the image formation apparatus of the fourth embodiment, therotary axis of the photosensitive drum or the driving roller (roller 906in the fourth embodiment) may be provided with an inertia load unit suchas a flywheel. By installing the inertia load unit such as the flywheel,it is possible to reduce load variations in the high-frequency area inthe photosensitive drum or the conveyor belt, and consequently toprovide a stable load variation correcting control operation or entiredriving. controlling operation; thus, it becomes possible to easilyprevent slipping. Here, in the case when the flywheel is attached to therotary axis of the photosensitive drum or the driving roller, if this isattached directly to the rotary axis of the photosensitive drum, theresulting defect is that the image formation apparatus becomes heavier.

In order to eliminate this defect, for example, as illustrated in FIG.32, a flywheel 1601 is indirectly attached to the photosensitive drum(photosensitive drum C901, in the Figure) through a torque transmittingroller 1602. In the structure shown in FIG. 32, as compared with a casein which the inertia load is attached to the same axis of thephotosensitive drum, the inertia moment of the flywheel 1601 viewed fromthe driving axis of the driving axis of the photosensitive drum isrepresented by a square of the radii of the photosensitive drum C901 andthe transmitting roller 1602. Therefore, a comparatively light weightflywheel can be used so as to obtain a required inertia moment so thatit is possible to achieve a light-weight image formation apparatus.

Moreover, with respect to the structure of the flywheel, the heavier theouter circumferential portion, the greater the inertia to be given tothe photosensitive drum. For this reason, the outer circumferentialportion of the flywheel is made thicker than the inner circumferentialportion thereof so that it is possible to obtain a lightweight apparatuswhile obtaining an inertia required.

Moreover, in the image formation apparatus of the fourth embodiment, therotation angle detecting encoder for detecting the rotation angle as anabsolute value may be installed in the photosensitive drum.Alternatively, in the image formation apparatus of the presentinvention, an encoder (belt shift amount detecting encoder), whichgenerates a pulse each time the conveyor belt 915 shifts a predeterminedlength by detecting the reference mark put on the conveyor belt 915using the leading position detector 909, may be installed.

Furthermore, a motor which is operated based upon a signal generated bythe rotation angle detecting encoder or the conveyor belt shift-amountdetecting encoder may be installed on the conveyor belt side. FIG. 33shows a construction in which, in place of the motor 907, a motor 1708that is a driving source for driving the conveyor belt is installed.Here, since the construction shown in FIG. 33 is virtually the same asthe construction shown in FIG. 25, those members that have the samestructures and functions as those of FIG. 25 are indicated by the samereference numbers, and the description thereof is omitted.

In the construction shown in FIG. 33, the motor 1708 is directlyconnected to the conveyor belt, with the result that the contact areabetween the conveyor belt 915 and the roller 906 becomes larger. Forthis reason, the permissible amount of the load variations imposed onthe conveyor belt is made greater. Moreover, in the image formationapparatus of the fourth embodiment, the roller 906 and the motor 1708may be indirectly connected through a gear.

Moreover, in the fourth embodiment, the load variation correcting motorsare placed on both of the photosensitive drum side and the conveyor beltside. However, the image formation apparatus of the present invention isnot limited to such a construction, and like the construction shown inFIG. 33, the load variation correcting motor may be placed only on thephotosensitive drum. In other words, the load variation correcting motoris installed on the side that has no motor driving the entire systembetween the photosensitive drum and the conveyor belt so that whilereducing an increase in the number of members to be added to the imageformation apparatus, the driving property of the conveyor belt can beimproved.

Here, in FIG. 25 or FIG. 33, the rotation angle detecting encoder isconnected to the photosensitive drum K904 or the belt driving roller 906that is driven by the motor 907, 1708 that drives the entire system. Inother words, the construction carries out the controlling operation byfeeding back a detector in the vicinity of the driving source. Thisconstruction makes it possible to reduce the possibility of any unstablemechanical factors (resonance or mechanical deviations) occurringbetween the driving source and the detector.

Moreover, the image formation apparatus of the fourth embodiment may beapplied to an image formation apparatus of a multiple-writing system forsimultaneously writing respective latent images on the respectivephotosensitive drums using a plurality of beams.

In accordance with the fourth embodiment as described above, it ispossible to provide an image formation apparatus which is lesssusceptible to slipping between the photosensitive drum and the conveyorbelt and can stably form images that are free from color offsets. Inother words, it becomes possible to provide an image formation apparatuswhich can provide image with high-quality that are free from coloroffsets and image distortion, even when there are load variationsimposed on the photosensitive drum and the conveyor belt due to acleaner, etc., and variations in the transfer position due toeccentricities in the photosensitive drum and deviations in the radiusthereof.

Here, when there is an eccentricity in the photosensitive drum, at laserexposing position E on the photosensitive member in FIG. 20 at which alatent image is formed through exposure made by a laser beam, etc., thelinear velocity is varied due to the eccentricity. The main scanning(which is a scanning method in a direction orthogonal to thesub-scanning method carried out in the rotation direction of thephotosensitive member) beam, which is applied onto the photosensitivedrum by the known polygon mirror, is applied out in a constant timeinterval.

This results in variations in the scanning pitch in the sub-scanningdirection formed on the photosensitive member due to the eccentricity.The above descriptions have discussed systems for reducing color offsetsand image distortion in an image. In other words, although the linedensity of the image formed by each photosensitive drum has deviations,the positional variations of the image thus formed are reduced.

In other words, although the above descriptions have provided means foreliminating the positional deviations in an image transferred from eachphotosensitive drum to a sheet of paper, no descriptions have been givenof the variations in the scanning pitch generated due to eccentricitiesin the photosensitive drum, that is, the deviations in the line density.In exposure position E in FIG. 20, the linear velocity (peripheralvelocity) LE in the tangential direction of the photosensitive drum ofthe photosensitive drum is virtually represented as follows:LE=eω cos β  (60)

In the case when a latent image formed at this point E is developed andtransferred at transfer position T₁, with respect to the linear velocityLT in the tangential direction of the photosensitive drum, therelationship, LT=LE, holds. In other words, when the linear velocity isLE, the resulting transferring process is carried out on the beltwithout slipping, no irregularities occur in the density in thetransferred image that has been exposed at exposure point E, and formed.That is, by providing a construction in which the contact portionbetween the photosensitive drum and the belt or a sheet of paper formsan apex in the belt direction on the circle cross-section of thephotosensitive drum and the exposure is carried out at point E, thedeviations in the density in the image are automatically corrected.

Since no other conditions provide the relationship LT=LE, it is notpossible to correct the deviations in the density in an image in anyother conditions. In the case when an exposing process is made at anexposure position other than point E, that is, at a position on anextended line from the rotary axis of the photosensitive drum that isshifted with an angle z from point E clockwise around the rotary axisthereof, the shift amount d(z) is represented by the following equation:d(z)=(π+z)(R ₀ −R)+R sin⁻¹[(ε/R)cos θ]  (61)When image data is generated in accordance with this data, it ispossible to correct color offsets and image distortion; however, it isnot possible to correct the line density. When provision is made so thatthe deviations in the line density, that is, the phases of variations,are made coincident with each other in transferred images on a sheet ofpaper that are formed by the respective photosensitive drums, therespective superposed colors are improved on a line basis, therebyproviding an image with high quality.

This is achieved by controlling the relationship of the eccentricpositions (ε, θ) among the photosensitive drums so that the respectivephases are made coincident with each other so as to allow the linevelocities on the exposure surfaces of the respective photosensitivedrums to be as close as possible, upon formation of the respectivelatent images (upon laser exposure on to the photosensitive drum) thatare to be superposed on a sheet of paper to form color images formed bythe respective photosensitive drums. In FIGS. 25 and 33, either the loadvariation correcting motor or the motor for driving the entire system isinstalled in each photosensitive drum. Moreover, as described earlier,the means for measuring the eccentric position is also provided therein.

In the construction shown in FIG. 25 or FIG. 33, although its entiresystem is not shown, the conveyor belt system including the rollers 906,905, 911, 912, 913 and the tension roller 914 is shifted downward, andthe contact between the conveyor belt and each photosensitive drum iseliminated so that the respective photosensitive drums are allowed tomove independently. Here, in order to allow the varied phases in theline density in latent images formed on the respective photosensitivedrums due to eccentricities in the photosensitive drums to coincide withone another on transferred images on a sheet of paper, the respectivephotosensitive drums are rotated and adjusted. Referring to FIG. 34, thefollowing description will discuss these operations.

In FIG. 34, on the assumption that the distance between thephotosensitive drum C and the photosensitive drum M is Dm, the distancebetween the photosensitive drum C and the photosensitive drum Y is Dy,the distance between the photosensitive drum C and the photosensitivedrum M is Dk, the radius of the photosensitive drum C is Rc, theeccentric position is (εc, θc), the radius of the photosensitive drum Mis Rm, the eccentric position is (εm, θm), the radius of thephotosensitive drum Y is Ry, the eccentric position is (εy, θy), theradius of the photosensitive drum K is Rk and the eccentric position is(εk, θk), in order to superpose an image formed by the photosensitivedrum M on an image on a sheet of paper formed by the photosensitive drumC, provision is made so that, after the image on the sheet of paperformed by the photosensitive drum C has moved the distance Dm, the imageformed by the photosensitive drum M is superposed thereon; and in thiscase, with respect to the phase difference θcm in the exposure timingbetween the photosensitive drum C and the photosensitive drum M,θcm=Dm/Rm holds.

In this case, when the phase difference in the eccentric positions isset to θcm, the linear velocities at the time of exposure on theexposing surfaces of the two photosensitive drums for forming images tobe superposed are made as close to each other as possible. Since thereare deviations in the respective drum diameters, it is impossible tomake the images formed by the two photosensitive drums completelycoincident with each other, and the deviations become greater as theposition in question proceeds from the leading end toward the rear end.In order to reduce the offset that occurs due to the deviations in thephotosensitive drum diameters, it is preferable to slightly correct θcmso as to have the minimum offset virtually in the center of the sheet ofpaper.

In the same manner, the phase difference θcy in the eccentric positionof the photosensitive drum Y is set to θcy=Dy/Ry, and the phasedifference θck in the eccentric position of the photosensitive drum K isset to θck=Dk/Rk. In other words, on the assumption that the measuredvalue of the angle position (eccentric angle) at the time of thegreatest eccentricity of the photosensitive drum C is θc, with respectto the photosensitive drums M, Y and K, the rotation angle positioncontrol is carried out so as to set as follows: the eccentric angle θmof the photosensitive drum M is set to θm=θc−θcm, the eccentric angle θyof the photosensitive drum Y is set to θy=θc−θcy, and the eccentricangle θk of the photosensitive drum K is set to θk=θc−θck.

Upon completion of these operations, the entire conveyor belt system isreturned to the original position to allow the photosensitive drums andthe conveyor belt to contact each other. In this case, when a rotationangle encoder for measuring the absolute value of the rotation angle isattached to all the photosensitive drums, the phases in eccentricitiescan be made coincident with each other by using the conventionalrotation angle positional control for driving the rotary motor whiledetecting the encoder output.

In another structural example in which, in place of the rotation angleencoder, a pulse generator (reference angular position detector) forgenerating one pulse each time the photosensitive drum rotates once isused, it is not possible to determine the angular position while feedingthe angular position back. In this case, in accordance with the rotationangle required for making a correction, a driving current as shown inFIG. 35 is supplied to the rotation motor so as to carry out thisoperation. 117.

The rotation angle shift amount is determined by changing the pulsewidth or the amplitude in FIG. 35. This method has been known as theBang-Bang control. After correction has been made using this method, theentire conveyor belt system is returned to the original position toallow the photosensitive drums and the conveyor belt to contact eachother, and the eccentric position is confirmed by using theabove-mentioned method, and when it is not within a target angularposition, the entire conveyor belt system is shifted downward, and theangular position correction is carried out by carrying out the sameoperations. By repeating these operations, the precision is improved.Thus, it is possible to reduce degradation in the image due toeccentricities in the respective photosensitive drums.

However, when there are deviations in the drum diameter, the phasegradually comes to have deviations as the printing or copying process isrepeated. These deviations can be corrected by carrying out theabove-mentioned operations so as to adjust the phases in the eccentricpositions of the respective photosensitive drums upon activation or shutdown of the printer or the copying machine, or upon completion of apredetermined number of prints or copies.

In the above-mentioned embodiments, explanations have been given ofconstrictions in which the photosensitive drums and the conveyor beltfor carrying out sheets of paper are provided. In other words, theexplanations have dealt with the system in which a toner image formed ona photosensitive drum is directly transferred from the photosensitivedrum to a sheet of paper. With respect to other structural examples, thesame descriptions are, of course, applied to a system in which tonerimages formed on respective photosensitive drums are transferred on abelt (intermediate transfer belt) to form a color image thereon withoutusing a sheet of paper, and this color image on the intermediatetransfer belt is transferred onto a sheet of paper by using the knownmethod. In this case, the same descriptions related to the transferringprocess on to paper can be applied to the transferring process onto theintermediate transfer belt. In other words, the same descriptionsrelated to the method for reducing color offsets or image distortion onthe belt and to the system for making the phases in the line densitycoincident with each other to properly superpose the respective colorsare applicable in the same manner.

Next, the following description will discuss fifth, sixth and seventhembodiments of an image formation apparatus in accordance with thepresent invention. In the image formation apparatus of the fifth, sixthand seventh embodiments, in addition to the image formation apparatusdescribed in the third embodiment, in order to reduce slipping betweenthe photosensitive drum and the conveyor belt, a motor, which drives torotate the conveyor belt driving motor in synchronism with a pulsegenerated by the rotation angle detecting encoder attached to thedriving roller supporting the conveyor belt each time the conveyor belthas moved a predetermined distance, is attached thereto. The thirdembodiment is applicable when no slipping exists between thephotosensitive drum and the belt or between the photosensitive drum andpaper.

The fifth, sixth and seventh embodiments have been devised based uponthe following ideas. In other words, what is taken into consideration isthat when a frictional force between the photosensitive drum and thebelt, between the photosensitive drum and paper, or between the paperand the conveyor belt, is small, slipping tends to occur. In otherwords, provision needs to be made so as to reduce control errors of theload variation correcting control system that provides control to cancelthe above-mentioned load variations by generating a force opposing tothe load variations imposed on the respective photosensitive drums. Inother words, the force in the belt shifting direction, imposed on theconveyor belt or a sheet of paper, that is generated due to controlerrors becomes greater than the frictional force, slipping occurs. Thefifth, sixth and seventh embodiments have been devised to deal withthese cases.

The fifth embodiment is provided with a load variation correctingcontrol system which detects a reverse electromotive force that is indirect proportion to the rotation velocity of the load variationcorrecting motor, compares this with a reference velocity signal fordetermining an appropriate rotation speed so as to provide controllingoperations. The sixth embodiment is provided with a load variationcorrecting control system which compares a phase difference between apulse that is generated once for each rotation of the photosensitivedrum with a reference pulse that has an appropriate pulse interval fordetermining a proper rotation velocity so as to provide controllingoperations in order to further improve the precision of the loadvariation correcting system as compared with the fifth embodiment.Moreover, the seventh embodiment provides a more stable load variationcorrecting control system with higher precision in combining the fifthembodiment or the sixth embodiment based upon the following principles.

An error in the load variation correcting control system forms a load tothe conveyor belt. Therefore, the current waveform flowing through theconveyor belt driving motor is monitored, and a judgment is made as towhether the correction of the load variation motor is excessive,insufficient, or proper; thus, a correction is made on the referencesignal (amplitude, pulse frequency, etc.) of the load correcting drivingmotor control system.

In other words, control errors to various load variations on aphotosensitive drum are transmitted as load variations in the conveyorbelt driving motor. In response to these load variations, the conveyorbelt driving motor is controlled to have constant rotations so that thecorresponding driving current flows through the conveyor belt drivingmotor. Therefore, when this is detected and fed back to the loadvariation correcting motor control system, it is possible to furtherreduce the load variations with respect to the entire driving control.

If the correction is not carried out successfully to cause a loadgreater than a frictional force between the photosensitive drum and theconveyor belt, slipping occurs. Therefore, the load variation correctingmotor control system controls in such a manner that among currentcomponents of the conveyor belt driving motor, a current to be appliedto its own system is not made greater than a predetermined value.However, with respect to the driving current flowing through theconveyor belt driving motor, it is not possible to tell which loadvariation correcting control system causes an error that corresponds tothe driving current in question. For this reason, an identifying sinewave (identifying multiplex sine wave) is multiplexed on the drivingcurrent of the load variation correcting motor. That is, the loadvariation correcting current is modulated.

In the case when this frequency is located on a band area higher thanthe controlling band area of the entire system driving motor, since theentire system driving motor is not controlled properly, the velocity ofthe conveyor belt is automatically varied by this frequency. When thisis within the controlling band area, the control system operates so asto correct the variation of this frequency, thereby allowing a currentto flow through the entire system driving motor. If there is asufficient gain in the entire system driving motor control system, it ispossible to ignore the influences to an image due to variations in theconveyor belt caused by this error. When the preliminarily determinedfrequency component of the current flowing through the entire systemdriving motor is detected, it is possible to detect the situation inwhich the load variation correcting motor is being controlled.

The current signal of the identifying multiplex sine wave must not causethe velocity of the photosensitive drum to vary and consequently to giveadverse effects on the image formation. It is preferable to make thedriving frequency of the identifying multiplex sine wave greater to adegree not to cause influences to the velocity variations in thephotosensitive drum; however, normally, this is very difficult. In otherwords, the frequency of the identifying multiplex sine wave has to beselected within the above-mentioned control band area of theentire-system driving motor. Therefore, the degree of modulation of thedriving current needs to be determined to be located within the levelnot to allow the velocity variations in the photosensitive drum causedby the current to give adverse effects on the image. In other words, thedegree of modulation is selected in such a manner that even if worstcomes worst, the velocity variations in the photosensitive drum arelimited to a level not to give adverse effects on the image. When thedegree of modulation has been determined, it is possible to measure whatloads are imposed on the entire system motor driving control system as awhole.

If the current frequency of the identifying multiplex sine wave in eachphotosensitive drum can be changed, it is possible to simultaneouslydetect what loads are imposed on the entire system motor driving controlsystem as a whole. In this case, it is necessary to have a sufficientlywide controlling band area in the entire driving system. Therefore, whensuch a band area is not available, the detection is carried out in atime-divided system. Moreover, the application of an arrangement inwhich the feedback is not always made and the detection is carried outin timing other than image forming operations is effective when thefriction between the photosensitive drum and the conveyer belt is not sogreat, that is, when the permissible control error is small. Forexample, this is executed prior to image formation or upon activation ofthe apparatus. When such timing other than the image forming mode isselected, the rotation velocity of the photosensitive drum may bevaried.

Moreover, in order to positively execute the above-mentioned processesprior to image formation, the amplitude of the reference signal of theload variation correcting control system (or the gain for allowing theamplitude to have an appropriate size), or the reference pulsefrequency, needs to be set to have high precision. In other words,provision needs to be made so as not to cause slipping at the time of animage formation. Therefore, first, the reference signal for the loadvariation correcting system is fixed, and the driving operationincluding the entire system driving motor is carried out. The referenceinput of the load variation correcting system (reference velocity signalin the load variation correcting system) has great errors since it hasnot been corrected. For this reason, the gain of the load variationcorrecting system is preliminarily made smaller. At this time, the loadon the periphery of the photosensitive drum is small since the imageforming process is not operated; therefore, conditions that wouldprovide sufficient driving operations can be selected. Then, in order tosuccessively obtain the amplitude of the reference signal input (or thegain for allowing the amplitude to have an appropriate size) of eachload variation correcting system, or the reference pulse frequency, theidentifying multiplex sine wave current is applied as described above soas to carry out an appropriate correction on the reference input. Uponcompletion of this correction on the four load variation correctingsystems, the apparatus is in a state ready for the image formation.

FIG. 36, etc. show a construction that realizes the above-mentionedideas. FIG. 36 is a drawing that explains an essential part of animage-forming apparatus of the fifth, sixth and seventh embodiments. Inthe same manner as the third embodiment, the image formation apparatusin the Figure, which is a tandem type image formation apparatus, isprovided with: a photosensitive drum C901, a photosensitive drum M902, aphotosensitive drum Y903 and a photosensitive drum K904. Moreover, theimage formation apparatus is provided with a conveyor belt 915, a roller905, a conveyor belt driving roller 906 for driving the conveyor belt915, a tension roller 914, rollers 911, 912 and 913. Above the conveyorbelt 915, a leading position detector 909 for detecting a referencemark, not shown, formed on the conveyor belt 915. As described earlierby reference to FIG. 22, this mark is detected so as to form a referencelatent image.

Here, the image formation apparatus of the fifth, sixth and seventhembodiments is also provided with an image-reading section such as ascanner, a paper-feeding section including paper-feeding cassettes, afixing section for fixing a toner image on a sheet of paper, apaper-discharging section and a corona charger. Moreover, each of thephotosensitive drum C901, the photosensitive drum M902, thephotosensitive drum Y903 and photosensitive drum K904 has a writing unitfor writing a latent image by scanning the surface with a laser light, adeveloping device for forming a toner image by supplying toner onto thelatent image, a cleaner, a static charger, etc. These structures arealso well-known structures; therefore, drawing indicating these andexplanations thereof are omitted.

Here, the developing device provided in the photosensitive drum C901supplies cyan toner, the developing device provided in thephotosensitive drum M902 supplies magenta toner, the developing deviceprovided in the photosensitive drum Y903 supplies yellow toner and thedeveloping device provided in the photosensitive drum K904 suppliesblack toner.

In the image formation apparatus shown in FIG. 36, the photosensitivedrum C901, the photosensitive drum M902, the photosensitive drum Y903and the photosensitive drum K904 are respectively provided with rotationangle reference position detectors 931, 932, 933 and 934. Moreover, theconveyor belt driving roller 906 for driving the conveyor belt isprovided with a rotation angle detecting encoder 908. The rotation anglereference position detectors 931, 932, 933 and 934 are provided with,for example, pulse detectors each of which generates a pulse each timethe photosensitive drum rotates once, and an encoder for generatingpulses the number of which corresponds to the shift amount of theconveyor belt is used as the rotation angle detecting encoder 908.

Moreover, the photosensitive drum C901, the photosensitive drum M902,the photosensitive drum Y903 and the photosensitive drum K904 arerespectively provided with load variation correcting motors 921, 922,923 and 924, and the conveyor belt driving roller 906 has a motor 907that is directly connected to the rotary axis.

The motor 907 is a motor that drives the conveyor belt 915 to rotate. Inother words, the motor 907 rotates to allow the conveyor belt drivingroller 906 to rotate so that the conveyor belt 915 contacting theconveyor belt driving roller 906 is shifted. In association with therotation of the conveyor belt 915, the rollers 905, 911, 912, 913 andthe tension roller 914 are allowed to rotate. Moreover, the loadvariation correcting motors 921, 922, 923 and 924 detect variations inthe loads imposed on the photosensitive drum C901, the photosensitivedrum M902, the photosensitive drum Y903 and the photosensitive drumK904, and the detected variations are subjected to the motor torque soas to reduce the load variations.

Here, the loads variations which are suppressed by the load variationcorrecting motors 921, 922, 923 and 924 are loads generated by acleaner, etc., located on the outer surface of the photosensitive drum.These loads tend to vary periodically. Here, with respect to the controlof the image formation apparatus by the rotation angle detecting encoder908 and the motor 907 and the operations of the load variationcorrecting motors 921, 922, 923 and 924, the description thereof will begiven later.

As illustrated in FIG. 26, each load variation correcting motor (theload variation correcting motor 924, in the Figure) can be attached tothe photosensitive drum (the photosensitive drum K904, in the Figure) ata portion outside an area in which a writing operation is performedthrough a comparatively small motor 1001. Thus, the application of theload variation correcting motor 924 makes it possible to increase themotor efficiency and to reduce power consumption, in comparison with thedirect connection of the motor to the photosensitive drum, so that it ispossible to use a smaller motor, and consequently to reduce the size ofthe image formation apparatus described in the fifth, sixth and seventhembodiments.

Moreover, in the image formation apparatus in the fifth, sixth andseventh embodiments, pulses generated by the rotation angle detectingencoder 908 are detected, and the measured values of the pulses arecompared with the rotation amounts of the photosensitive drums 901, 902,903 and 904 detected by the rotation angle reference position detectors931, 932, 933 and 934. Based upon these comparisons, it is possible todetect the rotation angle of each photosensitive drum that is made whilethe conveyor belt driving roller 906 rotates once in the image formationapparatus of the fifth, sixth and seventh embodiments. Alternatively, itis possible to detect the number of pulses generated by the rotationangle detecting encoder 908 while the photosensitive drum rotates once.

Furthermore, in the image formation apparatus of the fifth, sixth andseventh embodiments, which relates to the image formation apparatus thatuses a driving roller to shift the conveyor belt, the rotation amount ofthe conveyor belt driving roller 906 that is made while thephotosensitive drum rotates once is preliminarily stored. Then, therotation amount of the conveyor belt driving roller 906 thus stored iscompared with the detected rotation amount of the conveyor belt drivingroller 906, and when the difference of the two exceeds a permissiblerange, it is judged that the image formation apparatus is in an abnormalstate.

FIG. 27 is a block diagram that explains the controlling operation ofthe image formation apparatus carried out by the rotation angledetecting encoder 908 and the motor 907. The construction, shown in FIG.27, is provided with: the conveyor belt driving roller 906, the motor907 for rotating the conveyor belt driving roller 906, the rotationangle detecting encoder 908 for detecting the rotation angle of theconveyor belt driving roller 906, a control circuit 1100 for controllingthe motor 907 based upon a signal detected by the rotation angledetecting encoder 908, a power amplifier 1104, a phase compensatingdevice 1105, an f-V converter 1106 and an encoder pulse detector 1107.Moreover, the control circuit 1100 is provided with a phase comparator1101, a charge pump 1102, and LPF (Low Pass Filter) 1103.

When the velocity of the conveyor belt has reached a target velocity V,the rotation angle detecting encoder 908 outputs a pulse having afrequency of fr based upon the following equation. In other words,supposing that the target velocity of the conveyor belt is V and theradius of the conveyor belt driving roller 906 is Rr, the rotationvelocity ω_(r) of the entire-system driving motor is represented asfollows:ω_(r) =V/Rr  (62)

Moreover, supposing that the number of pulses that the encoder outputswhile the conveyor belt driving roller 906 rotates once is Nr, thefrequency fr of the pulse that the encoder outputs is represented asfollows:fr=Nr·ω _(r)/(2π)=Nr·V/(2πRr)  (63)The control section generates a signal S1 equal to this frequency fr,and inputs this to the phase comparator 1101.

The phase comparator 1101 receives a pulse signal S4 formed based upon apulse that the rotation angle detecting encoder 908 has actuallyoutputted, and compares this with the reference pulse S1 to calculate aphase difference between them. The calculated value is converted into avoltage signal that is represented in an analog format after passingthrough the charge pump 1102 and the LPF 1103, and further inputted tothe power amplifier 1104. Based upon the phase difference betweeninputted signal S1 and signal S4, the power amplifier 1104 outputs acontrol signal to the motor 907, thereby controlling the motor 907 so asto allow the conveyor belt 915 to shift at the target velocity V.Consequently, the motor 907 is allowed to rotate constantly with anangular velocity that is obtained when the conveyor belt 915 shifts atthe target velocity V. The above-mentioned process is a well-knownprocess, that is, a so-called PLL (Phase Locked Loop) process.

Here, the pulse, generated by the rotation angle detecting encoder 908,is outputted to the f-V converter 1106 through the encoder pulsedetector 1107. The f-V converter 1106 converts the pulse to a voltagesignal to generate a voltage signal S3 that is proportional to theangular velocity of the conveyor belt driving roller 906. This voltagesignal S3 is fed back to the power amplifier 1104 through the phasecompensating device 1105 so that the velocity controlling characteristicof the conveyor belt driving roller 906 is improved.

Moreover, since the voltage signal S3 is directly proportional to theangular velocity of the conveyor belt driving roller 906, this isoutputted to other constructions as the signal indicating the rotationvelocity of the conveyor belt driving roller 906 so as to be used forvelocity control in other systems constituted by the photosensitivedrums.

Moreover, variations in the load (magnitude, timing) imposed on theconveyor belt driving roller 906 or the conveyor belt 915 arepreliminarily found, and these variations in the load are appliedthereto from the external device as a feed forward signal S2. Such afeed forward control makes it possible to improve the velocity controlcharacteristic of the photosensitive drum even when no load variationcorrecting motor 924 is installed.

Here, the timing and magnitude of load variations imposed on thephotosensitive drum, the conveyor belt driving roller, or the conveyorbelt in an apparatus forming electronic photographs (a printer or acopying machine) are preliminarily known. Thus, the above-mentioned feedforward control is available. In the embodiments of FIG. 36, therotation angle detecting encoder 908 and the motor 907, directlyconnected to the axis of the conveyor belt driving roller 906, are used;however, these encoder and motor may be connected thereto through gears.

Next, an explanation will be given of the fifth embodiment of the imageformation apparatus. FIG. 37 is a block diagram that explains theconstruction for controlling the load variation correcting motor. Theconstruction, shown in the Figure, is provided with a drum-C loadvariation correcting control system 1200 for correcting the loadvariations in the photosensitive drum C901, a feed forward signalgenerator 1210, a drum-M load variation correcting control system 1206for correcting the load variations in the photosensitive drum M902, adrum-Y load variation correcting control system 1207 for correcting theload variations in the photosensitive drum Y903 and a drum-BK loadvariation correcting control system 1209 for correcting the loadvariations in the photosensitive drum K904.

Moreover, the drum-C load variation correcting control system 1200 isprovided with a current supply type power amplifier 1201, a phasecompensating device 1202, a load variation correcting motor 921, areverse electromotive force detector 1204 and a comparator 1205 for usein the reverse electromotive force detection.

The above-mentioned construction makes it possible to cancel the loadvariations in the photosensitive drum by using the torque of the loadvariation correcting motor 921, so as to minimize the influences thatare given by the load variations in the photosensitive drum to the otherphotosensitive drums through the conveyor belt.

The feed forward signal generator 1210 inputs signals for generatingtorques for canceling the known load variations (magnitude, timing)improved on the photosensitive drum C901, photosensitive drum M902,photosensitive drum Y903, photosensitive drum K904, that is, feedforward signals, into the corresponding control systems, that is, thedrum-C load variation correcting control system 1200, the drum-M loadvariation correcting control system 1206, the drum-Y load variationcorrecting control system 1207 and the drum-BK load variation correctingcontrol system 1209.

Moreover, the voltage signal S3 is respectively inputted to the drum-Cload variation correcting control system 1200, the drum-M load variationcorrecting control system 1206, the drum-Y load variation correctingcontrol system 1207 and the drum-BK load variation correcting controlsystem 1209. The signal S3 is a signal for determining the rotationvelocity of the load variation correcting motor. In this case, withrespect to the convey or belt load variation correcting control system,the signal S3 is converted to a value so as to allow the conveyor belt915 and the photosensitive drum to rotate integrally, thereby forming areference signal that determines the rotation of the motor.

Moreover, the reference signal for each of the photosensitive drums,inputted to the load variation correcting control system, may be formedinto a reference signal for determining the rotation of the motor, whichis obtained by taking into consideration the deviations in the radius ofthe photosensitive drum to make a correction. Furthermore, the drum-Cload variation correcting control system 1200 of FIG. 37 detects areverse electromotive force generated in proportion to the rotationvelocity of the load variation correcting motor 921, and compares thiswith the voltage signal S3 so as to control the velocity. In the fifthembodiment, the other load variation correcting control systems alsohave the same circuit construction. The load variation correctingcontrol system provides control so that even when there is a loadvariation in the speed determined by the reference signal fordetermining the rotation of the load variation correcting motor, therotation is maintained. In other words, consequently, the amount of theload variations that is transmitted to the other driving systems isreduced, with the result that it is possible to reduce slipping thatoccurs between the photosensitive drum and the conveyor belt or a sheetof paper.

In accordance with the above-mentioned operations, the load variationcorrecting motors 921, 922, 923 and 924 are controlled in response tothe rotation angular velocity of the motor 907 for driving the conveyorbelt 915. For this reason, the image formation apparatus of the fifthembodiment is provided with a construction in which even at the time ofactivation, the photosensitive drum is less susceptible to slipping (inwhich only either of the photosensitive drum and the conveyor belt 915is shifted) Here, the reverse electromotive force generated in the loadvariation correcting motor 921 can be detected by subtracting an innerresistance value from the voltage applied to the terminal. Moreover, inorder to improve the controlling characteristics of the constructionshown in FIG. 37, the current supply type power amplifier 1201 is usedas the power amplifier and the phase compensating device 1202 isinstalled.

For another construction example, the signal for determining therotation angular velocity of the load variation correcting motor may begenerated by a controller, not shown. Although other control modes suchas starting and stopping modes are not described in the fifthembodiment, these can be achieved by using the conventional technique.

As described above, in accordance with the image formation apparatus ofthe fifth embodiment, the driving system including the photosensitivedrum and the conveyor belt 915 is velocity-controlled by the motor 907,and the load variations imposed on the other photosensitive drums andthe conveyor belt 915 that are driven in accordance with the conveyorbelt driving roller 906 rotated by the motor 907 are corrected by theindividual control systems.

Next, an explanation will be given of the sixth embodiment of the imageformation apparatus. In the sixth embodiment, the apparatus in the fifthembodiment is further improved. In other words, this embodiment providesa method for setting the rotation velocity of the photosensitive drummore accurately so as to prevent slipping from occurring between thephotosensitive drum and the conveyer belt or a sheet of paper byutilizing the fact that if the conveyor belt moves at a constantvelocity, the photosensitive drum also moves at a constant angularvelocity. FIGS. 38 and 39 are block diagrams that explain theconstruction for controlling the load variation correcting motors 921,922, 923 and 924. The construction shown FIG. 38 includes a drum loadvariation correcting control section that detects the rotation state ofthe photosensitive drum, and allows the photosensitive drum to overcomeload variations imposed thereon in accordance with the state, therebycontrolling the photosensitive drum to have a constant angular velocity.The drum load variation correcting control section controls the loadvariation correcting motor to have a constant velocity, so as to allowthe torque of the load variation correcting motor to cancel the loadvariations imposed on the photosensitive drum. Since this arrangementregulates influences of the load variations to other photosensitivedrums, etc., and also reduces the load variations imposed on theconveyor belt or a sheet of paper, it becomes possible to preventslipping. Moreover, the construction shown in FIG. 39 is a fd generationsection for generates a clock frequency fd that serves as a referencebased on which the processes of the construction shown in FIG. 38 arecarried out.

The fd generation section, shown in FIG. 39, is provided with afrequency synthesizer 1301, a k(N+P) counter 1302, a PLL processingsection 1303 (constituted by k·No counter 1305 and a signal processingsection 1304 consisting of a phase comparator, a charge pump, a loopfilter and a VCO), and a controller 1306 for controlling theabove-mentioned construction.

The frequency synthesizer 1301 receives a pulse signal from anoscillator, not shown, that oscillates with a pulse oscillatingfrequency equal to the frequency fr of a pulse outputted from therotation angle detecting encoder 908 while the conveyor belt 915 isshifting at a velocity V. Then, based upon the frequency fr, itgenerates a pulse frequency fdo on the assumption that the rotationreference position detector of an ideal photosensitive drum outputsthis. Then, the PLL processing section 1303 and k(N+P) counter 1302generate a reference clock frequency fd for use in the load variationcorrecting control system of FIG. 38. The frequencies fdo and fd arepulse frequencies formed in accordance with the following relationalexpression.

When the motor 907 is driven to move the conveyor belt at a shiftingvelocity V, the photosensitive drum is allowed to rotate. At this time,the rotation angular velocity ω of the photosensitive drum isrepresented as follows:ω=V/R.When the respective photosensitive drums are rotated at the angularvelocity ω in accordance with this expression, it is supposed that noslipping occurs in any of the photosensitive drums. When the idealphotosensitive drum diameter is Ro, the number of output pulses No ofthe encoder at the time of one rotation of this photosensitive drum isrepresented by:No=Ro·Nr/Rr  (64).From the mechanical point of view, when Ro/Rr is set to a naturalnumber, a controlling operation with higher precision is obtained.

Upon measuring the actual radius of the photosensitive drum, in thepresent-embodiment, the pulses outputted by the rotation angle detectingencoder 908 at the time of one rotation of the photosensitive drum arecounted. Supposing that the number of pulses outputted at the time ofone rotation of the photosensitive drum is N and the phase indicatingthe distance between pulses is 2πP (where 0<P<1), the rotation angledetecting encoder 908 outputs pulses of N+P at the time of one rotationof the photosensitive drum. Therefore, the actual radius R of thephotosensitive drum is represented by:R=Rr(N+P)/Nr  (65).

In order to allow the photosensitive drum to move integrally with theconveyor belt having the velocity V, the rotation angular velocity ω isset as follow:ω=V/R=V·Nr/{Rr(N+P)}  (66).Here, the rotation velocity ω₀ of the photosensitive drum having anideal shape is ω₀=V/R₀ holds. Therefore, the following equation holds:ω={No/(N+P)}ω₀  (66)′.

At this time, the output pulse frequency fd of the rotation anglereference position detector 931 is represented by fd=ω/(2π), and theoutput pulse frequency fdo of the rotation angle reference positiondetector of the ideal-shape photosensitive drum is represented by:fdo=ω ₀/(2π),Therefore, fd={No/(N+P)}fdo  (67)The relationship between fr and fdo is represented by:fr=Nr·V/(2πRr)=(Nr·Ro/Rr)fdo  (68)fdo={Rr/(Nr·Ro)}fr  (69)

The frequency fdo, outputted by carrying out a frequency conversioncorresponding to equation (69) by using the frequency synthesizer 1301,is inputted to the k(N+P) counter 1302 through the PLL processingsection 1303. The controller 1306 calculates the frequency fd from thefrequency fdo corresponding to equation (67) by controlling the PLLprocessing section 1303 and the k(N+P) counter 1302. Here, k, shown inthe Figure, is a natural number, and is determined in accordance withthe detection precision of the rotation angular detecting encoder 908.For example, when the phase detecting resolution of the rotation angledetecting encoder 908 is 0.2×2π (⅕ cycle of pulse cycle), an appropriatefigure not less than 5 is selected as k.

The PLL processing section 1303 is a construction in which fdo issubjected to a multiplying process of k·No·fdo. The k(N+P) counter 1302is a pre-settable counter in which a count value can be set; thus, fd isfound by dividing k·No·fdo. Here, with respect to kP, a figure that isrounded off to a natural number is used. The frequency fd, calculated asdescribed above, is outputted to the drum load variation correctingcontrol section shown in FIG. 38.

The drum load variation correcting control section, shown in FIG. 38, isprovided with: a phase comparator 1307, a charge pump 1208, a loopfilter 1309, a controller 1306, a D-A converter 1407, a phasecompensating device 1204, a current supply type power amplifier 1201 anda load variation correcting motor 921. Here, the drum load variationcorrecting control section is installed in each of a plurality ofphotosensitive drums, and the construction shown in FIG. 38 belongs tothe photosensitive drum C901 of these.

The frequency fd, generated in the fd generation section, is inputted tothe phase comparator 1307 of the drum load variation correcting section.Moreover, a pulse signal, which is released from the rotation anglereference position detector 931 of the photosensitive drum correspondingto the drum load variation correcting control section, is inputted tothe phase comparator 1307. The drum load variation correcting controlsection compares the frequency of the inputted pulse signal and itsphase with the frequency fd and its phase, and controls the loadvariation correcting motor so as to make them coincident with each otherso that the respective photosensitive drums are rotated at a constantangular velocity.

The drum load variation correcting control section installed in eachphotosensitive drum allows the photosensitive drum to rotate at aconstant angular velocity ω, with the result that the photosensitivedrums installed in an image formation apparatus are controlled to have aproper velocity without causing slipping even when there areeccentricities and deviations in the radius.

Moreover, in the case when the amount and timing of load variationsimposed on the photosensitive drum or the conveyor belt 915 are known,the drum load variation correcting control section outputs a feedforward signal S6 in accordance with the amount and timing of the loadvariations from the controller 1306. Since this reduces the gainrequired for the feedback control system (so-called PLL control system)using the phase comparator 1307 of FIG. 38, it is possible to provide amore stable control system with higher precision.

Moreover, the drum load variation correcting control section detects asignal that is in proportion to the rotation velocity of thephotosensitive drum from the load variation correcting motor 921 byusing the reverse electromotive detector 1203, and this is fed back.This construction is designed to add a velocity feedback system bydetecting the signal in proportion to the rotation velocity of thephotosensitive drum from the load variation correcting motor so as toprovide a more stable controlling operation. Here, in the PLL system inFIG. 38, the controlling operation is made with respect to the pulseoutputted once for each rotation of the photosensitive drum; therefore,in an attempt to correct variations occurring within this pulseinterval, this velocity feedback system is added so that a more stablecontrolling operation with higher precision is obtained.

Moreover, the controller generates a signal S7 based upon the velocity ωcalculated from equation (24), and inputs this to the D-A converter1407. The resulting signal S8 that has been D-A converted in the D-Aconverter 1407 is compared with the reverse electromotive force detectedby the reverse electromotive force detector 1203, which is compensatedfor its phase in the phase compensating device 1204. Here, the reverseelectromotive force is detected by subtracting the inner resistancevalue r from the voltage at the terminal of the load variationcorrecting motor 921. In this case, the current supply type poweramplifier 1201 is used so as to improve the controlling property, andthe phase compensating device 1204 is interpolated so as to compensatefor the stability of the system.

The drum load variation correcting control section cancels the loadvariations imposed on the photosensitive drum based upon the results ofthe comparison, and controls the load variation correcting motor so asto allow the photosensitive drum to rotate at the angular velocity ω.Through the above-mentioned processes, the image formation apparatus ofthe present embodiment allows the motors 907 to control the entiresystem including the photosensitive drums and the conveyor belt, andalso allows the load variation correcting motors 921 respectivelyinstalled the drums to correct the load variations in the photosensitivedrums.

Next, an explanation will be given of the seventh embodiment of theimage formation apparatus. The seventh embodiment makes it possible tofurther improve the fifth and sixth embodiments. This embodiment iseffectively used in a system which, upon application of the fifth orsixth embodiment, is susceptible to slipping due to the resultingcontrol errors. In other words, in the case of a weak frictional forcebetween the photosensitive drum and the conveyor belt, between thephotosensitive drum and a sheet of paper, or between the sheet of paperand the conveyor belt, only slight control errors cause slipping.Therefore, the seventh embodiment provides a system having bettercontrol precision.

Referring to FIG. 40 and thereafter, the following description willdiscuss a construction that realizes the above-mentioned ideas. FIG. 40is a block diagram that explains the construction for controlling motorsin the seventh embodiment. Here, the mechanical structure of the imageformation apparatus of the seventh embodiment is the same as that of thefifth or sixth embodiment explained by reference to FIG. 36; therefore,those members of the seventh embodiment that correspond to those shownin the image formation apparatus of FIG. 36 are indicated by the samereference numbers, and the description thereof is omitted.

The construction shown in FIG. 40 indicates a drum C load variationcorrecting control system for correcting load variations in thephotosensitive drum C901. This control system is provided with: a sinewave oscillator 1401 for generating an identifying multiplex sine wave,an attenuator 1402 for attenuating an input signal, a multiplier 1403for multiplying signals outputted from the sine wave oscillator 1401 andthe attenuator 1402, a switch 1405 for switching on and off based upon asignal from a controller 1404, a current supply type power amplifier1406, a D-A converter 1407, a load variation correcting motor 921, arotation motor reverse electromotive force detector 1409, a comparator1410 and a variable gain circuit 1411.

FIG. 41 is a block diagram of a system 1510 for detecting a currentwaveform flowing through a motor for driving a conveyor belt drivingroller 906 in the control circuit of FIG. 27. A motor current detectioncircuit 1500 is a circuit for detecting a current flowing through themotor 907 for driving the conveyor belt driving roller 906. Moreover,this circuit is provided with: a band pass filter 1501 for allowing onlysignals having a predetermined band width of the system 1510 fordetecting the current waveform flowing through the motor, an absolutevalue circuit 1502, a low-pass filter 1503 for allowing only signals inthe low frequency band width to pass, a sample hold circuit A1504, asample hold circuit B1505, a sign detecting comparator 1506, and an A-Dconverter 1507.

FIGS. 41 and 40 show circuits to which the above-mentioned contents areadded to FIGS. 27 and 37 as specific embodiment. FIG. 40 shows a loadvariation correcting control system that relates to the photosensitivedrum, and FIG. 41 shows a current supply type power amplifier which isprovided in place of the power amplifier 1104 in FIG. 27 so as to detecta current that is proportional to the motor propelling force. FIG. 42 isa timing chart that explains these operations. Here, FIG. 38, whichshows another embodiment of FIG. 37, does not include the sine waveoscillator 1401, the attenuator 1402, the multiplier 1403, the switch1405, etc. shown in FIG. 40; however, as in the case of FIG. 37 to whichthese circuits are added to form FIG. 40, these circuits maybe addedthereto to obtain the same functions.

In the following embodiment, an explanation will be given of a case inwhich the operation for detecting the control errors of the respectiveload variation correcting control system is carried out in atime-divided system. When the entire control system is in operation, inan attempt to reduce transmission of the load variations to the entiredriving motor system of FIG. 27 caused by errors of the load variationcorrecting control system related to the photosensitive drums of FIG.40, the controller 1404 turns the switch 1405 on in FIG. 40, so as toprovide correction of the reference signal of the load variationcorrecting control system with high precision. In other words, to theinput of the current supply type power amplifier 1406 is inputted,together with a signal that has been inputted thereto before thiscontrol mode started, another signal formed by multiplying the resultingsignal derived from this signal attenuated by the attenuator 1402 by adetecting sine wave signal having a predetermined frequency generated bythe sine wave oscillator 1401. In other words, the signal, which is tobe transmitted to the entire driving motor side of FIG. 27 so as tomeasure the load variations, is modulated. Consequently, since thecorresponding current is allowed to flow through the load variationcorrecting motor 921, the corresponding sinusoidal propelling forcevariations occur therein.

In FIG. 41, the sine wave having the size corresponding to thetransmitted load variations is detected from the output of the motorcurrent detection circuit 1500 of the entire driving motor system. Asignal other than this sine wave, which gradually changes and containsDC, also contains the load variations transmitted from the otherphotosensitive drum systems; therefore, this is not used as controllinginformation. Moreover, the load variations caused by errors in the loadvariation correcting control systems attached to the respectivephotosensitive drums and the load variations imposed on the conveyorbelt are mainly composed of mechanical variations, and consequently,components on the high band area are small; therefore, by selecting thefrequency of the detecting sine wave is selectively set to the high bandarea side within the control band area of the entire driving motorsystem, the errors that are carried on the load variation detectionsignal in the above-mentioned frequency components detected by the motorcurrent detection circuit 1500 are small. The sin wave signal componentspecified by the band-pass filter 1501 is detected so that it ispossible to detect the high precision control error of the loadvariation correcting control system. After the control signal that isthe input of the current supply type power amplifier 1406 have beenmultiplied by a given rate (constantly attenuated), this is furthermultiplied by the sine wave; therefore, by detecting the amplitude ofthe output of this band-pass filter output, it is possible to estimatethe transmitted variation amount. This is based upon the fact that theinput voltage of the current supply type power amplifier 1406 isvirtually in proportion to the current flowing through the loadvariation correcting motor 921. In the case when the control errors inthe load variation control systems become greater, the load in theentire driving system of FIG. 27 becomes greater so that the amplitudeof the output of the band-pass filter 1501 becomes greater. Therefore,the amplitude is detected through the absolute value circuit 1502. Then,the signal detection is stabilized through the low-pass filter 1503.

However, at this time, even if the transmitted load amount is found, thedirection of the transmitted load is not known. In other words, it isnot known whether the errors in the load variation correcting systemsare exerted in the pulling direction with respect to the belt shiftingdirection or in the reversed direction. Therefore, in the timing in thesign detecting variable gain in FIG. 42, the gain of the variable gaincircuit 1411 for changing the reference signal to the control system inFIG. 40 is slightly changed by gain data from the controller 1404 sothat the reference signal is slightly changed, or the proper referencesignal frequency fd in the load variation correcting system and thereference velocity data S7 in FIG. 38 are slightly shifted, so as todetect the direction. For example, in the case when the shift is made inthe direction for reducing the velocity of the photosensitive drum, ifthe detected sine wave output becomes smaller in the entire drivingmotor current detection circuit outputs, it is found that the loadvariation control system is working in the pulling direction of thebelt. In other words, when it is working so as to pull the belt, thepulling force becomes weaker. Here, the output amplitude of the sinewave detection is within the predetermined range, it is not necessary tocarry out this operation. In other words, the apparatus is lesssusceptible to slipping in this case. In contrast, the application ofthe operation might cause problems such as misjudgment in the sign.

The sample hold circuit A1504 in FIG. 41 detects the gain of thevariable gain circuit 1411 of each of the load variation correctingsystems, or the output of the low-pass filter 1503 that is the amplitudeof the sine wave prior to the change in the reference signal frequencyfd or the reference velocity data, and the sample hold B1505 detects thegain of the variable gain circuit 1411, or the amplitude of the sinewave signal obtained after the reference signal frequency fd or thereference signal data has been changed. Then, the sign detectingcomparator 1506 is used so as to find the difference between the samplehold circuits so that the direction of the transmitted load is found. Inthe present embodiment, the transmitted load amount from the output ofthe sample hold circuit A1504 is estimated; however, the output of thesample hold circuit B1505 may be used. In the case of the output of thesample hold circuit A1504, since the information is obtained prior tothe change in the reference signal frequency fd or the referencevelocity data, a slight correction is required. The load variationtransmitted amount and its direction are measured in this manner;therefore, based upon these measured values, the controller 1404 setsthe gain of the variable gain circuit 1411 in FIG. 39 or the referencevelocity data S7 in FIG. 38, as well as setting the count values of thek(N+P) counter 1302 and k·No counter 1305 in FIG. 39, therebydetermining the reference signal frequency fd. In this case, the k·Nocounter 1305 is provided as a pre-settable counter so that the frequencyof the reference signal frequency fd is freely set in the increasing andreducing directions.

As described above, the load that is transmitted to the entire drivingmotor due to errors in the load variation correcting control system canbe reduced. With respect to the other load variation correcting systems,the same controlling operation can be carried out at different time.When the force transmitted to the conveyor belt due to the errors in theload variation correcting control is reduced, it is possible toeliminate slipping between the photosensitive drum and the conveyorbelt, between the photosensitive drum and a sheet of paper, or betweenthe sheet of paper and the conveyor belt.

Next, an explanation will be given of another structural example of theload variation correcting motor of the fifth, sixth and seventhembodiments. FIG. 29 is a drawing that shows an example of the loadvariation correcting motor in which: a roller 1304 is installed on therotary axis of the photosensitive drum K904, and the roller 1301, whichis directly rotated by the load variation correcting motor 1302, isallowed to contact the roller 1304 so as to drive this. Here, referencenumber 1303 represents an encoder. In the construction shown in FIG. 29,it is possible to improve the degree of freedom for the structure fortransmitting the driving force of the load variation correcting motor1302 to the photosensitive drum K904.

FIG. 30 is a drawing that shows an example of the load variationcorrecting motor that supports a roller 1401 rotated a the loadvariation correcting motor 1404 by using a spring 1402. In accordancewith the construction shown in FIG. 30, it is possible to sufficientlytransmit the driving force of the load variation correcting motor 1404to the photosensitive drum K904.

FIG. 31 is a structural example of the load variation correcting motorin which a disc-shaped transmitting member 1501 directly connected to aphotosensitive drum 1503 and a roller 1502 for stably transmitting thedriving force of a driving source 1504 are used. In the contact portionof the transmitting member 1501 and the roller 1502, the transmittingmember 1501 side has a flat face and the roller 1502 has a tapered shapeso as to increase the contact portion, thereby providing a stabletransmitting characteristic (characteristic for preventing slipping).The rotary axis of the driving source 1504 is not in parallel with thesurface orthogonal to the rotary axis of the transmitting member 1501.

Moreover, the present invention is not intended to be limited by theabove-mentioned embodiments. In other words, in the image formationapparatus of the fifth, sixth and seventh embodiments, the rotary axisof the photosensitive drums 901, 902, 903 and 904, or the rotary axis ofthe conveyor belt driving roller 906 may be provided with a flywheel;thus, it is possible to reduce load variations in the high-frequencyarea in the photosensitive drums or the conveyor belt, and consequentlyto provide a stable load variation correcting control operation orentire driving controlling operation. Consequently, it becomes possibleto easily prevent slipping. Here, in the case when the flywheel isattached to the rotary axis of the photosensitive drum or the drivingroller, if this is attached directly to the rotary axis of thephotosensitive drum, the resulting defect is that the image formationapparatus becomes heavier.

In order to eliminate this defect, for example, as illustrated in FIG.32, a flywheel 1601 is indirectly attached to the photosensitive drum(photosensitive drum C901, in the Figure) through a torque transmittingroller 1602. In the structure shown in FIG. 32, as compared with a casein which the inertia load is attached to the same axis of thephotosensitive drum, the inertia moment of the flywheel 1601 viewed fromthe driving axis of the driving axis of the photosensitive drum isrepresented by a square of the radii of the photosensitive drum C901 andthe transmitting roller 1602. Therefore, a comparatively light weightflywheel can be used so as to obtain a required inertia moment so thatit is possible to achieve a light-weight image formation apparatus.

Moreover, with respect to the structure of the flywheel, the heavier theouter circumferential portion, the greater the inertia to be given tothe photosensitive drum. For this reason, the outer circumferentialportion of the flywheel is made thicker than the inner circumferentialportion thereof so that it is possible to obtain a lightweight apparatuswhile obtaining an inertia required.

Moreover, in the image formation apparatus of the fifth, sixth andseventh embodiments, the rotation angle detecting encoder 908 fordetecting the rotation angle as an absolute value may be installed inthe conveyor belt driving roller 906. Alternatively, in the imageformation apparatus of the present invention, an encoder (belt shiftamount detecting encoder), which generates a pulse each time theconveyor belt 915 shifts a predetermined length by detecting thereference mark put on the conveyor belt 915 using the leading positiondetector 909, may be installed.

Moreover, in the fifth, sixth and seventh embodiments, the rotationangle detecting encoder 908 is attached to the conveyor belt drivingroller 906, and based upon the rotation angle detected by the rotationangle detecting encoder 908, the shifting velocity of the conveyor beltis controlled; however, for example, a rotation reference positiondetecting means or an encoder attached to the photosensitive drumdetects a velocity signal, and the shifting velocity of the conveyorbelt may be controlled by this signal.

Moreover, in the fifth, sixth and seventh embodiments, rollers are usedas the photosensitive drums, and an endless belt is used as the conveyorbelt; however, the present invention is not intended to be limited bythese; and the present invention may be applied to any system in which:at least one rotary member and a belt that is pressed onto the rotarymember and shifted are provided, and provision is made so as to operatethese integrally in a stable manner.

Moreover, the present invention may be applied to, for example, an imageformation apparatus of the multi-writing system in which a plurality ofbeams are used to write latent images onto respective photosensitivedrums.

In accordance with the fifth, sixth and seventh embodiments, it ispossible to provide an image formation apparatus which is lesssusceptible to slipping between the photosensitive drum and the conveyorbelt, and can provide an image that is free from color offsets in astable manner. This is because the present invention makes it possibleto provide a controlling method and a device thereof for accuratelycanceling loads imposed on the photosensitive drum and the conveyor beltwithout causing slipping.

As described above, in the present invention, based upon the amount ofeccentricity of the photosensitive drum, the eccentric rotation angleand the radius of the photosensitive drum, the distortion and coloroffset in the toner image that has been transferred are corrected sothat, even when high resolution is required for forming an image, it ispossible to sufficiently prevent the distortion and color offset in thesub-scanning direction of the image.

Moreover, the present invention makes it possible to precisely find anerror in transfer positions of toner images between the cases of theideal-shaped photosensitive drum and the actual photosensitive drum;therefore, even when high resolution is required for forming an image,it is possible to sufficiently prevent the distortion and color offsetin the sub-scanning direction of the image.

Moreover, the present invention makes it possible to precisely find anerror in transfer positions of toner images between the cases of theideal-shaped photosensitive drum and the actual photosensitive drum;therefore, even when high resolution is required for forming an image,it is possible to sufficiently prevent the distortion and color offsetin the sub-scanning direction of the image.

The present invention makes it possible to absorb errors in theassembling positions of the photosensitive drums, etc., therefore, evenwhen high resolution is required for forming an image, it is possible tosufficiently prevent the distortion and color offset in the sub-scanningdirection of the image.

Moreover, even when high resolution is required for forming an image,the present invention makes it possible to sufficiently prevent thedistortion and color offset in the sub-scanning direction of the image,by using an optical writing device having the same hardware structure asa conventional apparatus.

Furthermore, even when high resolution is required for forming an image,the present invention makes it possible to sufficiently prevent thedistortion and color offset in the sub-scanning direction of the image,by correcting image data using an optical writing device having the samehardware structure as a conventional apparatus.

Moreover, since main-scanning images that are scanned by a polygonmirror are stably outputted without interruption, it is possible toobtain an image with high quality.

Furthermore, it is possible to make deviations in the sub-scanning pitchdue to eccentricities in the photosensitive drum less conspicuous.

Moreover, main-scanning image data can be outputted in the same manneras in the case when there are neither eccentricities in thephotosensitive drum nor deviations in the radius of the drum diameter.

Even when there are eccentricities in the photosensitive drum, if thetransporting member is shifted at a constant velocity, the rotationangle of the photosensitive drum is made constant; therefore, theshifting operations of the photosensitive drum and the transportingmember can be detected by a single sensor so that it is possible to cutproduction costs, and also to make correction data for correctingdistortion and color offsets in the sub-scanning direction of an imagewith high precision.

Moreover, it is possible to reduce degradation in an image due tovariations in the sub-scanning pitch of an electrostatic latent image,caused by variations in the linear velocity at the exposure position ofeach of the photosensitive drums due to eccentricities thereof.

Furthermore, the angular velocity of the rotary member such as aphotosensitive drum is maintained constant more positively. Since loadvariations transmitted to the belt are reduced, it is possible toprevent slipping that might occur between the rotary member such as aphotosensitive drum and a sheet of paper or the belt. Therefore, whenthis device is applied to an image formation apparatus, it becomespossible to form an image free from offsets with higher image quality.

Moreover, it is possible to prevent an increase in the number of parts,and consequently to prevent the image formation apparatus, etc. becomingbulky.

It is possible to miniaturize the load correcting means for the rotarymember such as a photosensitive drum or the belt load correcting means.

Moreover, the load variations imposed on the rotary member such as aphotosensitive drum can be corrected for each of the rotary members suchas photosensitive drums; therefore, it is possible to reduce thedislocation of the rotary member such as a photosensitive drum with highprecision, and also to prevent slipping that might occur between therotary member such as a photosensitive drum and a sheet of paper or thebelt. Therefore, when this device is applied to an image formationapparatus, it becomes possible to form an image free from offsets withhigher image quality.

Moreover, it is possible to control the driving process of an imageformation apparatus more stably. For this reason, when this device isapplied to an image formation apparatus, it becomes possible to form animage free from offsets with higher image quality.

Furthermore, since the high-frequency components of the load variationscan be reduced, it is possible to improve the controlling property.Therefore, when this device is applied to an image formation apparatus,it becomes possible to form an image with higher image quality.

Since the high-frequency components of the load variations can beeliminated with a small inertia load, it is possible to prevent theimage formation apparatus from becoming bulky.

Moreover, even when there are eccentricities in the rotary member, ifthe belt is shifted at a constant velocity, the rotation angularvelocity of the rotary member such as a photosensitive drum is madeconstant. Therefore, in the case when a plurality of rotary members areinstalled, the rotation angle detection of the rotary members or thebelt shift detection can be carried out at only any one portion that isallowed to have a detecting function, and the rotation angle of therotary member or the belt or the shift position of the other members canbe estimated.

Moreover, it is possible to prevent slipping that might occur betweenthe photosensitive drum and the belt or a sheet of paper, and even whenthere are eccentricities in the photosensitive drum or deviations in theradius thereof, if the transporting member is shifted at a constantvelocity, the rotation angle of the photosensitive drum is madeconstant; therefore, it is possible to easily form an image that is freefrom image distortion or color offsets.

Moreover, based upon information related to eccentricities, informationrelated to the radius of the photosensitive drum or information relatedto the distance between the respective photosensitive drums, a pluralityof photosensitive drums can be positioned independently; therefore, thevaried phases in the line density of latent images that occur uponapplication of an exposing beam on the photosensitive drum with aconstant interval due to the eccentricities in the photosensitive drumscan be made coincident with each other when colors are superposed,thereby making it possible to improve the quality of a color image.

Moreover, material for preventing slipping between the photosensitivedrum and the belt, the photosensitive drum and a sheet of paper, or thesheet of paper and the conveyor belt, and means for solving the problemof irregularities in density in an image are used in a shared manner;therefore, it is possible to achieve high-quality images without causinghigh costs.

Furthermore, simply by selecting the exposure position, theirregularities in density in an image can be automatically corrected;thus, since no additional mechanisms are needed, it is possible toachieve high-quality images without causing high costs.

It is possible to provide means which, even when at least one rotarymember and a belt is provided and there are load variations in them,allows these members to move smoothly in an integral manner, and which,even when there are eccentricities in the rotary member, allows the beltto move stably at a constant speed, and when this means is applied to animage formation apparatus, it is possible to provide an improved imageformation apparatus and a controlling method for an image formationapparatus in which, independent of the states of respective imageformation apparatuses, such as eccentricities due to deviations in thephotosensitive drum at the time of assembling and deviations in theradius of the photosensitive drum, respective toner images aretransferred on a sheet of paper without causing positional offsets toform an image with high quality.

Moreover, since the control unit controls the operations of thedriving-roller drive unit and the rotary member driving unit inaccordance with load variations in the belt, the rotary member and thebelt are respectively controlled and driven based upon the loadtransmitted from the rotary member to the belt; therefore, it ispossible to eliminate slipping of the belt. The resulting imageformation apparatus is readily applied to a printing process with highimage quality.

Moreover, since the detection is carried out without the necessity ofany special devices, the load imposed on the belt can be detected byusing simple devices.

Furthermore, simply by detecting the current supplied to thedriving-roller drive unit, the load imposed on the belt can be detected;therefore, the load of the belt can be detected by using simple devices.

Since the identifying signal generation unit for generating anidentifying signal that makes it easier to detect the load variations inthe belt, it is possible to easily detect which rotary member generatesthe load variations in question simply by using a simple structure.

Moreover, since a sine wave is used as the identifying signal, it ispossible to easily detect which rotary member generates the loadvariations in question simply by using a simple structure.

Furthermore, since the identifying signal is applied in different timingto each of the rotary members, it is possible to easily identify theload variations for each rotary member.

Moreover, since the sine wave has a different frequency for each of therotary members, it is possible to easily identify the load variations inthe respective rotary members.

Furthermore, an inertial applying unit which applies inertia load toeither of the driving-roller drive unit and the rotary member drivingunit is installed; therefore, it is possible to lower the frequency bandin the velocity variations in the belt caused by the load variations,and consequently to provide a stable controlling operation for furtherreducing slipping on the belt.

Moreover, even when there are deviations in the shape of each rotarymember, it is possible to prevent the occurrence of slipping between therotary member and the belt, between the rotary member and a sheet ofpaper, or between the sheet of paper and the belt.

Moreover, even when there are deviations in the radius of each rotarymember, it is possible to prevent the occurrence of slipping between therotary member and the belt, between the rotary member and a sheet ofpaper, or between the sheet of paper and the belt.

Furthermore, without the necessity of installing a new radius measuringunit for the rotary member, the existing means for detecting therotation angle and the velocity and information from the means formeasuring the distance of the shift of the belt and for detecting thebelt shifting velocity are used for generating a velocity controllingreference signal; therefore, it is possible to provide a simpler imageformation apparatus, etc.

Furthermore, even when the number of pulses from the velocity detectoris one for each rotation upon detection by the rotation angle, therotary member velocity reference signal, which is compared with ananalog signal successively outputted from the velocity detection unit ofthe rotary member, is generated so as to provide a controllingoperation. In other words, the influences due to the deviations in theradium of the rotary member can be reduced by comparing the pulsesignals, and a stable controlling operation can be achieved by comparingthe analog signals; therefore, it is possible to provide a stable rotarymember velocity controlling operation for preventing slipping on thebelt.

Moreover, since the control errors of the rotary member is detected as aload directly imposed on the belt, the correction can be carried outwith higher precision; thus, it is possible to provide an apparatus thatis less susceptible to slipping.

Furthermore, even when there are eccentricities of the rotary member ordeviations in the diameter, the above-mentioned controlling operation isachieved with high precision; therefore, it is possible to achieve astable apparatus that is free from slipping.

Thus, it is possible to form an image that is free from image distortionor color offsets stably.

Since the operations of the driving roller and the rotary member arecontrolled in response to the load variations in the belt, the rotarymember and the belt are driven and controlled based upon the loadtransmitted from the rotary member to the belt, it is possible toeliminate slipping on the belt. In the image formation method, it isreadily applied to a printing process with high image quality.

Furthermore, simply by detecting the current supplied to thedriving-roller drive stage, the load imposed on the belt can bedetected; therefore, the load of the belt can be detected by usingsimple devices.

Since the identifying signal that makes it easier to detect the loadvariations in the belt is generated, it is possible to easily detectwhich rotary member generates the load variations in question simply byusing a simple structure.

Moreover, since a sine wave is used as the identifying signal, it ispossible to easily detect which rotary member generates the loadvariations in question simply by using a simple structure.

Furthermore, since the identifying signal is applied in different timingto each of the rotary members, it is possible to easily identify theload variations for each rotary member.

Moreover, since the sine wave has a different frequency for each of therotary members, it is possible to easily identify the load variations inthe respective rotary members.

Furthermore, a stage for applying an inertia load to either of thedriving-roller drive unit and the rotary member driving unit isprovided; therefore, it is possible to lower the frequency band in thevelocity variations in the belt caused by the load variations, andconsequently to provide a stable controlling operation for furtherreducing slipping on the belt.

Moreover, even when there are deviations in the shape of each rotarymember, it is possible to prevent the occurrence of slipping between therotary member and the belt, between the rotary member and a sheet ofpaper, or between the sheet of paper and the belt.

Moreover, even when there are deviations in the radius of each rotarymember, it is possible to prevent the occurrence of slipping between therotary member and the belt, between the rotary member and a sheet ofpaper, or between the sheet of paper and the belt.

Furthermore, without the necessity of installing a new radius measuringunit for the rotary member, the existing means for detecting therotation angle and the velocity and information from the means formeasuring the distance of the shift of the belt and for detecting thebelt shifting velocity are used for generating a velocity controllingreference signal; therefore, it is possible to provide a simpler imageformation method.

Furthermore, even when the number of pulses from the velocity detectoris one for each rotation upon detection by the rotation angle, therotary member velocity reference signal, which is compared with ananalog signal successively outputted from the velocity detection stageof the rotary member, is generated so as to provide a controllingoperation. In other words, the influences due to the deviations in theradium of the rotary member can be reduced by comparing the pulsesignals, and a stable controlling operation can be achieved by comparingthe analog signals; therefore, it is possible to provide a stable rotarymember velocity controlling operation for preventing slipping on thebelt.

Moreover, since the control errors of the rotary member is detected as aload directly imposed on the belt, the correction can be carried outwith higher precision; thus, it is possible to provide a stage that isless susceptible to slipping.

The present document incorporates by reference the entire contents ofJapanese priority document, 2000-86014 filed in Japan on Mar. 27, 2000,2000-156933 filed in Japan on May 26, 2000, 2000-169516 filed in Japanon Jun. 6, 2000, and 2000-204531 filed in Japan on Jul. 6, 2000.

Although the invention has been described with respect to a specificembodiment for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

1. An image formation apparatus comprising: at least one photosensitivedrum; an optical writing device which carries out an optical writingprocess in a main-scanning direction on the outer surface of the atleast one photosensitive drum at least line by line; a developing devicewhich develops an electrostatic latent image optically written on the atleast one photosensitive drum into a toner image by using toner; a paperconveyor belt which transfers a sheet to receive the toner image locatedon the at least one photosensitive drum, that is shifted in synchronismwith the at least one photosensitive drum while being pressed onto theat least one photosensitive drum; a rotation angle detection unit whichdetects a rotation angle of the at least one photosensitive drumdirectly or indirectly; an eccentricity detection unit which detects aneccentric position from the rotary axis of the at least onephotosensitive drum located in the center of a circle cross-section ofthe at least one photosensitive drum; and a correction unit which, basedupon results of detection by the rotation angle detection unit and theeccentricity detection unit, finds an amount of eccentricity of the atleast one photosensitive drum, an eccentric rotation angle and a radiusof the at least one photosensitive drum, and based upon resultingvalues, compensates for a distortion and a color offset of a toner imagebefore the transfer to a sheet.
 2. The image formation apparatusaccording to claim 1, wherein the correction unit finds an error betweentoner images at transfer positions of the sheet on the paper conveyorbelt after a transferring process, the toner images being respectivelyderived from the at least one photosensitive drum in an ideal shape andthe at least one photosensitive drum in an actual shape, and based uponthe error, compensates for a distortion and a color offset of a tonerimage before the transfer to the sheet.
 3. The image formation apparatusaccording to claim 2, wherein supposing that a shift amount of anexposed image from an ideal state thereof is d, the eccentric amount isε, the eccentric rotation angle is θ, the radius of the at least onephotosensitive drum is R and a radius of the at least one photosensitivedrum at the time when the at least one photosensitive drum has an idealshape is R₀, an exposure position on the at least one photosensitivedrum having the ideal shape is located with an angle (π−z) in the drumrotation direction of the at least one photosensitive drum from thetransfer position of the at least one photosensitive drum of the idealshape, with respect to the exposure position on the at least onephotosensitive drum by the optical writing device, said shift amount dis found based upon the following equation:d=π(R ₀ −R)+R sin⁻¹ {(ε/R)cos θ}+z(R ₀ −R).
 4. The image formationapparatus according to claim 1, wherein the correction unit detects anerror between installation positions of the at least one photosensitivedrum and at least one of the optical writing device, developing deviceand paper conveyor belt, and based upon the error, corrects a distortionand a color offset in the toner image.
 5. The image formation apparatusaccording to claim 1, wherein the correction unit selects appropriatedata from image data that modulates an exposing light beam outputtedfrom the optical writing device and controls the data so as to correct adistortion and a color offset in the toner image.
 6. The image formationapparatus according to claim 5, wherein an optical writing position onthe at least one photosensitive drum in a sub-scanning direction isfixed, and among interpolation image data that has been obtained bysubjecting an original image data to a positional interpolation processat least either in the sub-scanning direction or in the main-scanningdirection, the correction unit selects appropriate data and carries outa controlling operation.
 7. The image formation apparatus according toclaim 6, wherein the optical writing device main-scans the at least onephotosensitive drum by deflecting a light beam released from a lightsource by using a polygon mirror that is driven to rotate at a constantvelocity by a polygon motor, with the optical writing position beingfixed on the at least one photosensitive drum, and the correction unitoutputs the image data in synchronized timing with a start ofmain-scanning by the polygon mirror.
 8. The image formation apparatusaccording to claim 7, wherein the correction unit carries out an opticalwriting operation by changing optical recording conditions in responseto the peripheral velocity of the at least one photosensitive drum. 9.The image formation apparatus according to claim 5, wherein thecorrection unit controls an optical writing position on the outersurface of the at least one photosensitive drum so as not to select theappropriate data of image data for modulating the exposing beamoutputted by the optical writing device.
 10. The image formationapparatus according to claim 9, wherein a contact portion between thepaper conveyor belt and the at least one photosensitive drum forms anapex on a round cross section of the at least one photosensitive drum inthe paper conveyor belt direction.
 11. The image formation apparatusaccording to claim 9, wherein the paper conveyor belt presses a sheet ofpaper onto the at least one photosensitive drum to transfer the tonerimage formed on the at least one photosensitive drum.
 12. The imageformation apparatus according to claim 1, wherein a contact portionbetween the paper conveyor belt, the sheet or the paper conveyor beltand the photosensitive drum forms an apex on a round cross section ofthe at least one photosensitive drum in an paper conveyor beltdirection.
 13. The image formation apparatus according to claim 1,wherein the at least one photosensitive drum comprises a plurality ofphotosensitive drums aligned in one row on the paper conveyor belt inthe shifting direction of the paper conveyor belt, and when toner imagesrespectively formed on the photosensitive drums are transferred andsuperposed, the phases between the eccentricities of the photosensitivedrums and positions of electrostatic latent images respectively formedon the photosensitive drums are made virtually coincident with eachother with respect to the photosensitive drums so that the phases invariations in density of transferred images caused by the eccentricitiesare made coincident with respect to the photosensitive drums.
 14. Theimage formation apparatus according to claim 1, further comprising animage reading device which reads an image of an original, wherein imageformation is carried out based upon the image data thus read.